Again, they did allow Sakurai at least the flexibility to choose between Decidueye and Incineroar and there were other options provided alongside Greninja. How strict they are on the matter may not be as much as you imply.
There's two answers on this. The first is from
Sourcegaming and says that Incineroar wasn't chosen until at least after SM.
The second is from
NintendoEverything which says he was chosen as late as USUM's release.
I'm more inclined to lean to the former, but even so that would still put his inclusion after SM and thus would follow things like the Mimikyu short and early reception to Pokemon like Lycanrocs.