The entire evolution animation is too long to have in a trailer, and it would end up being boring to watch. By just including the very start of the evolution animation, viewers can understand that Pikachu evolves into Alolan Raichu.I still have some hope because they showed normal pikachu evolving, but not actually into that form of raichu. Why wouldn't they just show the complete evolution sequence if they weren't hiding something...
I don't remember trailers for Pokemon games dabbling in much misdirection, so I don't think they are hiding anything.