My theory, so that version of Quicksilver in the xmen was taking place in the 80's, and everything that comes into Wanda's show matches the time period, so I think between the 80s time period and multiverse they will use that to explain why this version of him is there. Not a full intro of the xmen and mutants into the MCU yet, but just a wink and nod that they can return to at another time. Or I could be totally off base lol.