So I think we can all agree that USUM makes it clear that Ultra Beasts are not /actually/ "legendary" pokemon, but an entirely new class of pokemon that simply gets lumped together with other legendaries under the "Sublegend" category in the game's code because they share the share restrictions when it comes to battle facilities/tournaments/etc. Similarly to how mythicals and "main" legendaries are together in the "Legend" category, despite being explicitly said to be different groups by TPCi.