The reliable PokeBeach source, which has been proven correct on everything so far, said Mawile will evolve and also become Steel/Fairy.
They never mentioned an evolution.
Indeed, that was a separate (and false) leak that said Mawile was evolving.
Oh, it could certainly evolve. Just that the leak that said it was happened to be fake. --- Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere