afaik, GF works with two teams, the Main team and the Remake team. In Gen 5, BW2 development started just after BW was finished, and it was handled by the remake team while the Main team started working on XY. Even before XY was released, the remake team was already working on ORAS. The Main tem is already working on the development of the Switch game that's not supposed to be here for at least a year and a half, while I bet the remake team started working on USUM as soon as SM was finished, the same way it happened in gen 5.