On any normal day before, this would be insane crackpot speculation. This time, we do have something real to latch onto.
I think the way to look at it is to realize that Sakurai has trolled from time to time. Sure, he's not always trolling, or he changes his mind on different aspects but I'm pretty sure even looking back at Fighter Pass 1, a few things always made me think we were not gonna see more DLC, Sakurai seemed eager to be getting close to the end, they said the budget for their presentations wasn't high, then... sike! More fighters in development. All of a sudden, Sakurai seems happy and invigorated to keep making fighters. It was such a whiplash.
In this video, he has a little troll right from the beginning. Sakurai then says he's gonna show who'll be on the roster this time. Right after that, they essentially start leading you on with characters until they just drop that everyone is coming back this time. They like building up hype. Lowering your expectations just a little to then give you something nice is appreciated.
They don't just walk in and say that everyone is coming back and also RIdley will be in. They reveal they're bringing in new fighters, tell us to not expect too many new ones..... but here's one right now! Ridley!
Since this could quite possibly be the end, they don't want people to be expecting the Third Pass at all. Having no real plans for DLC past what they're working on and not knowing whats next doesn't mean they couldn't possibly be saving one little surprise they've been working on in secret. Right??
Considering the oddly suspicious feeling comment on there being only two opportunities for presentations, it either feels like a purposeful misdirect for later, or just confirmation that we only have two Sakurai Presents left. Having one more bonus seems about right and not necessarily a lie. It just makes me feel the bonus may be a bunch of echo fighters instead of one new fighter.
Plant got no presentation, or little overview, and was just talked about during a Smash direct. This new bonus could quite literally be tacked on at the end of the last Sakurai presents for CP11.
"Alright that's everything... but we do have one more thing for those who've purchased both fighter passes... It's a Mii Costume for the new Grey Pikmin from Pikmin GO. Alright bye! Never ask me for anything ever again."