Also conspicuous is that Regigigas's current signature move didn't get snapped, even though Wring Out (which is a special move that's otherwise functionally identical) did. Won't be surprised if they end up changing its effect.
(For what it's worth, I can totally imagine some lore where the original Regigigas is incomplete, and the new one's a full-on Normal type box legendary. I don't think we have one yet, aside from Arceus, which may or may not make it in at some point.)
Arceus isn't a Box Legendary (technically it's not a Legendary, it's a Mythical Pokemon). A Box Legendary refers to the Legendary Pokemon that appears on a game box: Ho-Oh, Lugia, Suicune (who is more of an exception to the rule), Groudon, Kyogre, Rayquaza, Dialga, Palkia, Giratina, Reshiram, Zekrom, Kyurem, Xerneas, Yveltal, Solgaleo, Lunala, Necrozma, Zacian & Zamazenta. It's also up in the air whether we should consider Urshifu and Calyrex part of this as they're technically the "Box" Legendary for the Expansions (at the very least the logo is based on them).
Another technicality: Regigigas is nearly/actually as strong as a "normal" Box Legendary. Aside from special cases like Suicune, Kyurem, and Necrozma, the Box Legendaries have a BST of 680 (except for Groudon & Kyogre which had their's lowered to 670 so that Rayquaza could be the master of that group). Regigigas's BST is 670, but it's Ability cuts its Attack & Speed in half for five turns (which I guess makes it as if its BST is 540 as it loses 80 is Attack and 50 in Speed).
As for Regigiga being incomplete, that doesn't make sense. Lore-wise Regigigas came first. People feared its great power (legends said it had the strength to move continents) so sealed it away and created the Regi Golems to act as keys to awaken it. That's why the two new regis are a mystery, essentially some group created two more keys but for what purpose? Were they trying to awaken Regigigas but didn't want to look around for the Regi Trio so tried creating their own? Or could they be keys to a great power that Regigigas can use, maybe the original power it had so that it could move continents, which goes back to my Gigantamax suggestion.