The more I think about it, the more I’m coming to regard using X-Pietro as an excellent piece of misdirection on the part of Marvel. In-universe, Agatha cast some rando to play the part of Pietro. Now if Marvel had really cast some rando to play Fietro then we the audience would pretty much instantly have seen right through it. Putting in X-Pietro garnered shock, confusion and then acceptance from the audience that he really was Pietro, much like Wanda was initially confused then accepted that he was Pietro. And so Marvel trolled all the socials while still making Fietro fit into the storyline.