Honestly I think porygon-z was just an oversight. It's like 3 rematch teams deep into BDSP and the first two are her Platinum team.
And as for the BDSP location data you're a bit off the mark. While I don't know why Home is the way it is, Legends Arceus actually does account for it.
BDSP pokemon in LA are from "Sinnoh region made new" to mirror LA pokemon in BDSP being "Sinnoh region of old" while DP pokemon in LA are simply "Sinnoh Region" presumably because it would be weird to say "Good ol' Sinnoh" in the game with...old Sinnoh.
BDSP not differentiating between DPPt and BDSP feels more like, well, an ILCA thing honestly.....
Anyway I'm definitely not reading that much active malice into this.
There’s another reason why BDSP doesn’t recognized as it’s own Pokédex distinct from DPPt; the differences between Pokédexes are too insignificant to really count as different categories.
ORAS’ Pokédex is different from RSE’s Pokémon for one major key factor; it adds cross-generational evolutions from Gen 4 for Pokémon that are available in the original RSE, as such Dusklops can evolve into Dusknoir, without needing to obtain the National Dex firsthand. This alone makes ORAS’ Pokédex distinct from the Hoenn of old, hence the seperation between them between ”Hoenn” for ORAS and “Good Old Hoenn” for RSE.
ORAS’ prior remakes, FRLG and HGSS, do not make cross-gen evolutions and pre-evolutions available until the National Dex is obtained - though erronously in HGSS, Aipom, Lickitung, Yanma, Piloswine and Tangela can still evolve via a certain move if traded from DPPt beforehand - thus there’s no use for making FRLG and HGSS be their own Pokédex from the original versions.
Let’s Go Pikachu and Let’s Go Eevee repeated what FRLG did and going as far as making cross-gen evos and pre-evos not coded in the game at all, and the only new additions are the Meltan line. And even then, Meltan and Melmetal were only obtained from transfer from Pokémon GO through Pokémon HOME, so it’s not something the Kanto Pokédex took into account in the long run, unlike ORAS where NPC trainers, including Gym Leaders, do use the Gen 4’s cross-gen evolutions where applicable. An example is Phoebe using Dusknoir over one of her Dusklops.
BDSP is based on either Diamond and Pearl or Platinum, and doesn’t include Sylveon from Gen 6, thus no point of making BDSP’s Pokédex distinct from DPPt. Legends: Arceus, on the other hand, not only have it’s own few whole new (or in this case, rediscovered in-lore) Pokémon that is involved in the main game and post game without transfer, but also have Pokémon not available to capture in-game without trading in both DPPt and BDSP.
Legends: Arceus have by far the most Mythical Pokémon available to freely capture without time expiration… though if you manage to get BDSP and SwSh games, Shaymin and Darkrai are also available, making ALL Gen 4 Mythical Pokémon available to capture without online event, period, something unheard off. While not super disticnt by itself, it definitely gives L:A another distinction over DPPt and BDSP dex-wise, since the goal of L:A is to complete Hisui’s Pokédex by capturing almost all Pokémon (Darkrai and Shaymin are understandably optional) before taking on Arceus‘ avatar.
i unironically forgot the nerf of King's Shield. Though I'm not sure how much that actually affected the strenght of the defensive sets, I assume that did play a part on it.
