Vergeben said he had multiple sources confirm Incineroar, hence why he didn't commit to which Pokémon it was at first, rather than making a statement based on just one source.
That said, mistakes could always be made. Liam Robertson was convinced years ago that Shovel Knight would be in Smash, because his source learned of the Shovel Knight amiibo, and made the jump in logic that it meant he was in Smash (since only Smash amiibo existed at that point).
But multiple people making that error seems unlikely. We'll see.
I would have more faith in the Grinch leak (man I love that name) if Isabelle hadn't been announced. Because before her, it seemed like every character added was targeting the Smash fanbase specifically, with all these big requests being added. The Grinch leak is more of that, and I didn't believe in Incineroar being in the game for that reason; he's not really a request like all the other fighters. But Isabelle doesn't fit within that criteria, so that's opened up the possibilities quite a bit, so Incineroar seems more likely.
What if both leaks are wrong? What kind of chaos would that unleash?