I'm very confused by the fact they have an entirely new graphics engine yet the models and animations of Gen 1-6 Pokémon are exactly the same. Or is having a new graphics engine unrelated to having the same models and animations as before? I don't really know much about this sort of thing
They appear to re-use as much of their old work as possible. Makes sense, really. Between all the formes and all the Pokémon, there are some nine hundred models with textures and animations which were made for X/Y. Easily thousands of man-hours of work. Remaking it all for SuMo would cost a lot of development resources, so it's understandable why they just ported as much as posssible. Even with a new graphics engine, it's not like re-made models would have looked a lot better.
They've done this before, too. If I recall correctly, certain Pokémon models and animations from
Pokémon Stadium were used all the way up to Pokémon Battle Revolution. Not sure if they spruced up the textures, but either way, that's almost a decade and a half of using the same ground work.