You also have to remember, Ultra Sun and Moon are set in an Alternate Universe, so anything can happen.  Also, on Serebii, where I found the second video, they say some Pokémon are getting new Z-moves.

EDIT: Just checked Serebii again and saw they updated with another video, showing a new Z-move

https://www.youtube.com/watch?time_continue=63&v=oZevIAuNvoE