This is of course speculation, but in all honesty piecing everything together, especially with the recent announcement that Gen 9's debut games Scarlet and Violet still hitting stores for Holiday 2022 despite PLA coming out earlier this year, I am fairly confident that BDSP in and of itself was an afterthought and was not intended to even exist in the first place, or at least what I'm trying to say is that the BDSP we see today, the "Brilliant Diamond and Shining Pearl" games we have right now, were not meant to ever be a thing and were a last minute afterthought that they felt they "needed" to do after several factors led to changes in plans.
I think Game Freak originally had plans design Sinnoh remakes that looked more like Sword and Shield as the follow-up Gen 8 games on the Switch, and these concept arts may have been remnants of an intended DP-remake that looked more like Sword and Shield (and actually developed by Game Freak lol), that they were originally planning to do. I remember a rumored leak even before Gen 8 was announced that a person who leaked info said during the Gen 7 days that Gen 8 would be on the Switch and have two games: a base pair of games called Sword and Shield, and then Sinnoh remakes (under a tentative name) following, which if said leak is to be seen as a genuine leak at the time, which could've been true considering they got the Gen 8 games being named Sword and Shield correct, would imply that they had Gen 8 planned out long before it started and to have Sinnoh remakes serve as the second "Gen 8" games.
And then at some point when the time came to actually do the Sinnoh remakes they planned, they likely decided to change direction and turn what would've been the FRLG, HGSS, and ORAS of Sinnoh into a Sinnoh "pre-make": a game set in Sinnoh of past, with a new story and experience. Possibly because they might have thought that the Sinnoh people knew from Diamond, Pearl, and Platinum would be unrecognizable in a proper reimagining a la HGSS and ORAS in the modern Gen 8 engine, with a largely grid-based game with an intricate map design that worked specifically with 2D in mind that has, mind you, more routes than Sword and Shield and more landmarks, and they would've had to have some way to incorporate the Wild Area concept into it too without taking what made Sinnoh and ruining it. Given how different the design philosophies of Galar and DPP Sinnoh are there might've been some...issues there. Also perhaps because they wanted to pull their philosophy of surprising people with something new: Game Freak probably thought "Yeah people saw FRLG, HGSS, and ORAS and expect us to do the same for Diamond and Pearl now, but instead we should pull our surprises. Remember how they expected Pokemon Gray back in the Black and White days, but we surprised them with Black 2 and White 2? Or how they expected Pokemon Z but we gave them ORAS instead? Or how they expected Pokemon Stars, but we gave them Ultra Sun and Ultra Moon? Let's do the same this time, since they're all expecting DP remakes, but we should do a Sinnoh 'Pre-Make' instead!". Maybe it was a mix of both of those lines of thought.
Either way, this led to a change in plans and turned what would've been a remake of Diamond and Pearl into a completely new game: a "pre-make" of Sinnoh that does the usual remake job of reimagining an older region in a modern engine and updating the game to modern standards, so reimagines Sinnoh in the Sword and Shield engine and artstyle, while completely altering the experience and gameplay to align with Gen 8, including taking the Wild Area concept from SwSh and turning it into the entire game for this. In other words, it ended up being what we now know as Legends: Arceus. All the while, this new direction for a past-gen revisit allowed them to also take some bold steps and different changes in gameplay that they felt would be fun, with how different PLA's gameplay is from past ones. Instead of a "Diamond and Pearl" remake that would be constrained to align with Gen 8 philosophies, this game would make itself the Gen 8 game but become a totally new experience in doing so, and they figured a Sinnoh in the Gen 8 engine would be very different from Gen 4 Sinnoh anyway, so might as well go all the way with it!
I also strongly believe that PLA was by all means supposed to be the intended Holiday 2021 release, and that it was supposed to be released in November 2021 and serve as the 2021 game for the series, with the upcoming Scarlet and Violet serving as the 2022 game. However, it may have been delayed by a few months, probably because the arise of the COVID pandemic in early 2020 threw a bit of a wrench in PLA's development, and so because of that sudden worldwide issue arising the development was hampered by a few months so PLA ultimately got pushed a few months to the beginning of 2022, unable to make it in time for the holidays. Since they couldn't get PLA to release in time for 2021 Holiday season, combined with the vastly different direction PLA was taking compared to FRLG, HGSS, and ORAS, they had to get *something* out there to appease the corporate higher-ups and also sponge the social need for a "remake" since PLA wouldn't immediately register in people's heads as the true Sinnoh revisit a la HGSS and ORAS. So they basically contracted ILCA out to do a hard faithful "remake of Diamond and Pearl" with hard faithful map design, and make everything exactly as it was in DP so that they could say they had something to sponge the need for a Gen 4 remake, and appease the corporate overlords in the Pokemon business machine for the holidays to generate holiday sales, all the while it would absorb the need for a Sinnoh remake and tide fans over until PLA finally released a few months later and revealed itself as the true "Sinnoh remake" in the vein of FRLG, HGSS, and ORAS, the real next Gen 8 after Sword and Shield, and the true 2021 game for the series. This decision likely arose around 2020 after COVID threw a wrench in things and given the business schedule and whatnot they had ILCA do BDSP as faithfully and as hard as possible in around a two-year timeframe, which is likely why the end product ended up being so...well...sloppy...to say the least.
In other words, had things gone as planned for them, this would've been the release schedule over the past few years:
2019: Sword and Shield
2020: Sword and Shield DLC
2021: Legends: Arceus (which is what we ended up getting as our "Sinnoh Remake", but either way this would've been the Holiday 2021 game and the true 25th anniversary project and "BDSP" would straight up never have happened)
2022: Scarlet and Violet
Long story short and a TL;DR, BDSP was very likely an afterthought, the BDSP we know now was created as a last minute decision, DP remakes were originally supposed to look more like SwSh but then turned into Legends: Arceus, which is the real Sinnoh in SwSh style we have now, PLA was supposed to be a November 2021 release but was pushed back due to COVID, they needed to appease the corporate overlords to get a "Holiday release" out there but PLA couldn't make it so they contracted an outside company at the last minute to do a hard faithful "remake of DP" at the last minute and said "remake of Diamond and Pearl" was done within less than 2 years which is why it's as sloppy as it is now. The concept art we're seeing here may have originally been what would've been the DP remakes originally before they turned into PLA, if they are to be indicative of anything, and ended up in the BDSP artbook since that actually...uh...has the locations of modern Sinnoh despite not doing much with them graphically sadly.