Justifications: They kind of look alike, they're both depicted as having been betrayed by humanity and then exacting revenge upon it (Arceus movie and Ultra games). Arceus's body reacting to the Z crystals is very sus considering they're fragments of necrozma. Both are depicted being worshipped in their respective world (Legends Arceus and Pokemon SM anime) t's never been explained whether Ultra Space was created by Arceus or is its own thing so that would clear it up.