>>36899713Sakurai literally said this years ago before Smash 4's release. He talks with The Pokemon Company, and they choose a Pokemon that's going to be prominent in the anime.
Smash, the anime, it's all planned out ahead of time.
When the roster was chosen in late 2015, Incineroar was already on it. He was on the roster even before his model sheet leaked alongside Decidueye and Primarina.