Yuugis Black Magician
Namaikina Imouto
They used the XY art style because that's what they're used to from the past three movies, and the nod to the OS outfit and hat is what stands out the most.
I am not saying that there is nothing to reference other generations/series, but they're the kind of details that most people - especially kids - wouldn't notice. Again, aside from the ending and the lesser roles given to a few Sinnoh and Alola critters. If you want a movie with a giant ensemble of Pokemon, watch one of the XY ones.
石小百合 (Ishi'ichi Sayuri) is the chief animation supervisor and character designer for Movie #20. She held this role for the original series, Advanced Generation and did Design Draft for Diamond & Pearl. I imagine she was asked to return to the role because the movie was paying tribute to the first series and this is their weird way of doing that even though she isn't particularly outstanding at her job. The movie doesn't particularly look like Hiro'oka Toshihito's character designs for XY outside of the wonky body proportions he had.