I think I can (maybe) answer this question, and it's an idea I've seen some get close to but never go all the way with. Strap in though, because this'll be a long one, and the scope prevents a concise TL;DR.
First of all, absent official confirmation, we have been told by quite a few anonymous sources that unlike Smash 4, Nintendo/Sakurai are really cracking down on leaks this time around. Most have said over the past few months that Nintendo has been sending out bait to snuff out leakers, which seems to have worked a couple of times.
Before we continue there is something important to cover, and that is how different the landscape is now from 4 years ago. In the years since the ESRB leak Nintendo has gotten really buddy-buddy with fan channels, Gamexplain is a great example. We've also seen the rise of insiders/journalists/leakers/"shills" if you will, who have connections with individuals within companies, which have gotten wind of this.
With that in mind let me present a hypothetical. What if, in addition to sending some fake info to snuff out leaks. Nintendo has also created fake info and sent out this fake info to compromised sectors?
Think of this as the "Death Star II hypothesis". For context, in Star Wars: Return of the Jedi Emperor Palpatine sent out fake info about the status of the new Death Star's super weapon, (specifically that it was non-functional and that the shield generator on the moon of Endor was virtually non guarded, but in reality the weapon was operational and the Empire was fully expecting the rebels) and the rebellion took the bait, especially as Palpatine made the effort to get the information as grueling as it would have been had it been real.
So we could see a sequence of events as follows:
1. Sakurai finalizes a roster for SSBU
2. Sakurai/Nintendo want to prevent leaks, so they "leak" information of their own. In this scenario, banner leak is real so here Incineroar is fake information right from the source.
3. Information gets out just like any other leak (contacts within companies typically), except here it's information that is purposely misleading.
4. "Insiders" get this info, who are able to validate that it originated from Nintendo (because it did) and see no reason to believe otherwise.
5. Info spreads around to reporters like Vergeben and the like, who are also eventually convinced of its authenticity.
The beauty of this, if true of course, is that just like Palpatine's grand plan the false info came from an official source, so there would be no way for journalists to know about whether it was true or not, nor would there be a reason to question it.
What I don't think Nintendo was counting on though was somebody for whatever motivation they had making an Snapchat video of a printed poster (remember, this hypothesis requires us to assume the leak's legitimacy rather than falsehood, and that the person making the video did not design the poster, merely preparing it for an upcoming event, along with Grinch material for whatever that was for). Then said video got screenshoted by friends, one of whom leaked it online.
Nintendo now needs to do damage control, and gets the loose networks to double down on the legitimacy of the false info, resulting in all the confusion with the insiders. Thus the situation we have right now.
Before finishing this post, I would like to make one more observation for now about the background that I haven't really seen properly addressed. I can't remember when, but awhile back I was reading about how an image taken in motion can blur and distort an already burry and distorted image. Add in different lighting to this and you start to lose details that should be there. What I'm getting at is perhaps why things like waterfalls are missing is because of the moving camera, and that if we actually saw the mural in the flesh, we'd see the details that currently seem to be missing.