How the gamepress know the stats of unreleased pokemon?

Niantic uses a formula, it isn't random.

All of the Mythical Pokemon seem to have the same stats. (Jirachi, Mew, Celebi)

Also a tidbit of information:

Celebi is horrible, as it can't defend and it's under the dumpster fire that is Mewtwo, Alakazam, Espeon, Metagross, Exeggcutor and Lugia.

https://pokemongo.gamepress.gg/pokemon-stat-change

Here is the explanation for the stat conversion formulas.