I mean, you could always ask the writers if it's canon. Has there been an interview where they've said the X books happen in a different universe? How did the X-men get to the 616? Did the Inhumans patent the mist while the x-men and t-mist were dimension hopping? The questions are endless!
I don't understand, how is EXM not canon? The X-Men being in limbo has been referenced a lot.