I'm good with that explanation for Nulls, personally. They aren't legendary Pokemon in the sense of Arceus, even if they were modeled on Arceus, nor are they like Mewtwo who's truly one of a kind. It's perfectly reasonable to allow them as bootleg copies, especially when no character IC is going to have any idea they're 'supposed' to be a pseudo-legendary (and we do allow characters to have pseudos and have many running around in-game so I don't quite see what the issue is that other people are having with this). If anything I would assume the common thinking would be that they're like Porygon 3.0. I do like the idea of having the move type work like Hidden Power and be set according to player choice as well. (tearsofademon brings up a great point below that this game has pretty much never adhered to canon anyway lmao. I mean Alolan Pokemon are showing up on a completely unrelated island chain???)

Also this is just me editing to say that if we ever do have a plot of any beastly variety I'm here and I'm ready to have Ford Pines get absorbed by a Nihilego for research purposes. I'm ready. Please dear god.