This is a reasonable assumption to make. There is certainly something strange going on with this leak. I primarily believe it's the lack of Incineroar.
I'll say this now: Incineroar is in the game. Anyone with a fraction of insider knowledge knows this, which is why multiple people have come out and said it, including Mystic, who specifically said he's only saying what he's heard from multiple sources. It's just as much a well-known secret as Isabelle was before her reveal.
No, Incineroar is not leaker bait. The point of leaker bait is to catch leakers. It's in the name, bait. Why would Nintendo go and tell every insider's source that Incineroar is in the game when he isn't? The idea of it being a misinformation campaign is plausible, but what does that accomplish? Nintendo isn't able to pinpoint who's saying what if they tell everyone the same misinformation. It'd make more sense if we heard multiple Pokémon, but instead it's just Incineroar. I also doubt Nintendo would start a misinformation campaign right now in an attempt at damage control. They've proven to just ignore actual leaks, so why would they act so differently this time? And before you say "but Smash is special" and "they're handling leaks differently this time," we have no proof of that other than what boils down to PR statements. Even then, aspects of the E3 reveal got leaked even when Sakurai said they didn't, so their "leak protection" isn't as great as they say it is.
So, if Incineroar is in the game, why isn't he on the banner? That's the biggest question, and I'd say that's why insiders are doubting it. There is every reason to believe Incineroar is in this game. I believe some insiders believe that this leak is fake solely because it lacks Incineroar, who is a glaringly obvious omission. If this is the full banner, then why are characters missing?
Maybe it's not necessarily fake, but if it isn't, this is not the complete truth.
Look, if I can't argue conspiracy, you can't either.