With Season 2 of Legion still without a solid release date, it could legitimately be Stevens doesn’t know, or that talks are still happening.
With the Shadow King still in the world is one of the first mutants Xavier ever met in his travels, and has psychic abilities comparable to that of Xavier.
Perhaps Dan Stevens is playing coy, although if ever there were a time to make a major announcement regarding Patrick Stewart joining Legion, one would imagine San Diego Comic Con is the place to do so.