I'm not sure about XY because in the games Sycamore said that there were examples of mega evolutions only in the Kalos region, and Dexio said that Fairy type was recently just classified.
Well not opposed to the idea of being a multiverse, I'm still doubtful the ORAS and XY are in their own time line because it was posted by Matsumiya on twitter that BW2 and XY take place roughly the same time.
If needed, I can try to find a source I can post.