Yep, Sakurai himself said it some time ago. Though it should be noted that, as said before, Greninja's an outlier as he was picked before it was even a consideration, so how much of an influence is dependent on the case.
As for the main point, I will admit, this is something I've been thinking about lately. As much as we all consider Decidueye the frontrunner of Gen 7, we do tend to ignore that promotion-wise (and popularity-wise as well) there are at least three other candidates that we don't give a lot of credit to.
-In terms of I Choose You, Incineroar was clearly the favored starter here as it appeared under a rival and had two major fights with Ash that played a role in his development. Lycanroc was also his secondary and even got a minor moment of connection with his trainer towards the end, establishing that the Pokémon is also important to him, at least to an extent.
-In terms of the main anime, even ignoring Litten's evolution and Incineroar's debut, Litten has been built up more in terms of its story and strength relative to its counterparts and that could hint that the anime intended to have it evolve from the beginning and be the main starter of the group. Additionally, Ash's Lycanroc and Jessie's Mimikyu have also been heavily featured, and can be argued to be their respective aces at the current time.
-In terms of USUM promotion, all of the starters were actually pretty even. All three had a place in one of the major art features of promotion and neither of the three really stood out from one another. The regular Pokémon on the other hand, Lycanroc got a new form, a new Z-Move, and was present in advertising, Mimikyu got all except a new form, and a couple other Pokémon such as Kommo-o got a Z-Move.
-Spinoffs are admittedly very limited (looking at Serebii, there's only four this generation, two of which are ported from the previous one and one is a mobile game), but here Decidueye does win out since it was the only Gen 7 Pokémon picked for a fighter in Pokken DX. The other two were completely absent except in their base stage forms as assists.
In terms of Gen 7 Pokémon they can pick, Decidueye is not the only one that TCPi does push. Incineroar has gotten love, Lycanroc has gotten love, and Mimikyu has gotten love. Theoretically, if Sakurai did come to TCPi and talked with them on which Pokémon they should go with, it wouldn't be as much of a straight answer as we expect, you could make the argument for any of the above Pokémon.