Occam's Razor states that the simplest explanation is usually the correct explanation. What we have is what we have, it was big mistake to leave it in. Gamefreak made an error somewhere in there when they scrubbed everything else but the images.
That being said, this is the company that once added a Mew after their game went gold before just because they could. They also did something similar with a few design changes between Ruby/Sapphire if I recall correctly Chimeco was a late addition and Shellos and Gastrodon were removed, which is confirmed I believe.
What bothers me is the math here; 78 of those Pokemon from the datamine are brand new, not counting Alolan forms - including the Oricorio (4), WishiWashi (2) and Lycanroc (2) forms. Of those 78, 55 of them were previously seen, along with all but six Alolan forms and five ultra beasts.
Counting Alolan forms and Ultra Beasts, (18 and 8, respectively) you have a total of 104 new Pokemon (maybe, if Ultra Beasts are Pokemon were still not sure) in the game, but 69 of those Pokemon were already shown to us, including forms. Throw in Ash-Pikachu and Ash-Greninja, you bump that up to 106.
For a comparison, Generation 4 had a similar number, 107 new Pokemon, and pre-release, we had 23 Pokemon I believe known before Launch? We also had a similar trend, out of the 107 new Pokemon, 26 were a pre-evo or final evo for older generations, which matches up with the Alolan forms somewhat.
The choice to reveal the majority of the Pokedex, if this is the final count, is an interesting one. Perhaps the focus, in the end, is the side-missions and post-game content in the end over a high number of new Pokemon. Either way, what is interesting about this entire thing is the potential for more to be revealed, we still need names of course. We still need types and stats to go through. Will it happen? Will we see that soon, along with a surprise Pokemon or two?
Well, Occam's Razor states otherwise...but stranger things may happen.