Like how Froakie got in over Greninja?
There's no way in hell Rowlet would get in over Decidueye, who is absolutely a solid choice.
Your comparison is idiotic, Opossum.
How Greninja got in and how Decidueye would hypothetically get in are two different situations completely.
Neither Froakie nor Greninja were even
known about when Greninja was decided for Smash, and Greninja was picked out of all Kalos Pokémon solely for its design and moveset potential.
If Greninja hadn't been in Smash already and Alola wasn't a thing, Greninja would easily be the choice
now either way due to the promotion it has had. It'd have fit all the "criteria" Sakurai gave for Pokémon selection that it ironically was one of the only exceptions for due to the circumstances of inclusion.
Decidueye would be decided upon when Sun & Moon have already been released, which is a major strike against it in comparison to Greninja. Now it has to contend with whichever Pokémon are appearing in movies or the anime as well as the ones that are bigger deals for the 7th Generation as a whole.
And guess what?
That's where Rowlet comes in.
Rowlet is pretty much one of the two biggest "hot topic" breakout Pokémon from Sun & Moon, the other being Mimikyu.
Rowlet is also one that is getting heavy marketing. In comparison to not just the other two starters, but
its entire evolutionary line as well. For example, it had its own line of merchandise that's sold particularly well (Rowlet's Garden) and is even getting its own line of figures based off of Ash's Rowlet in the anime.
And it's not like Rowlet is short on the potential for a unique fighting style; Decidueye is not the ONLY ONE in the line that can stand out from the rest of Smash's cast.
So using logic and what Sakurai has said in the past in regards to how Pokémon are chosen, Rowlet is much more likely than Decidueye.
And in general a leak that has Decidueye is automatically fake until we're actually given reason to believe it is more likely than the likes of Marshadow, Incineroar, Lycanroc (Midnight), Tapu Koko, Mimikyu, Bewear, or even Rowlet itself.
EDIT: And we may end up adding Necrozma to that list as well based on recent reveals.