So, for someone who stills confused about all this shit:

It seems that NIANTIC updated these two models instead of Kecleon's models (Which is not available in GO yet)

The name of the pictures indicates that this Pokémon is no.891 on Pokédex. Still no confirmation about this Pokémon instead of its Pokédex number.

It MIGHT be LGPE's mythical, it MIGHT be a Gen VIII Mythical and it MIGHT be just an aleatory mid-dex Gen VIII Pokémon (I'd guess it's a first stage Pokémon)

The guy who "leaked" didn't say anytime about this being LGPE's "Very Special Pokémon", the first source implying this was Serebii.net

So, what is confirmed until now?
It's a Pokémon, not a placeholder, GF never used placeholders for shinies. (Remember SM's first leak when they put placeholder for Pokémon but didn't for shinies?
Its dex number is 891
The models were accidentally updated

What is NOT confirmed
It's a Mythical
It's LGPE Very Special Pokémon
It's just a random first stage Pokémon
Its Gen. Even being no.891 we don't know the final game it'll be so it may suffer some changes