When has that ever happened?
Adding extra Pokémon to official games? Let me see.
Easily done in gens I and II by using pokered and pokecrystal. In gen 3 it has been done for about 10 years now but used to involve tons of hex editing and other non beginner friendly tools, now that pokeruby and pokeemerald have taken off it seems it is much easier.
In gen 4 it has been done using again hex editors and general sprite editing tools.
From gen V on things got more complicated. Inserting a mon in BW isn't so much more difficult than in DPt but the problem is animating the sprite correctly.
In gen VI I've never seen anyone expand the dex but I've known of adding more Pokémon by (ironically in the case of this discussion) inserting it in another mon's slot, which used to be the first step towards actually expanding the dex in older gens. The hardest part is actually modelling and animating a custom creation, but since we have the models of the stuff we wanna add it might be simpler.
I never followed the scene in gen VII but it's safe to say it hasn't been done yet. However, sciresm tweeted about looking into it. I'm not sure if he was serious about it, but this person is responsible for magnethax (3DS hardware exploit that never was fixed), hacking the Switch early on, developing the Switch's first custom firmware and datamining most Pokémon games, but I'm not sure when exactly he begun doing that. But pretty much every learnset, stat, breeding list and such you find in Pokémon fansites was unearthed due to him or other hackers in his circle.
I honestly wouldn't doubt the dude in anything. At worst, he'll help us find out what do these models have that's given GF so much trouble.