I think the natural hubaloo and meaning being Peters being cast means he can’t be just “some dude” badly impersonating Pietro; the casting is too meta and the in-universe believability of his “impression” too lacking for it to be that. I mean, what, Agatha couldn’t find someone who looks more like the actual Pietro, so she just pulled a random athletic dude with pale hair, gave him powers, then almost immediately started leaning into how little he resembled the real deal on purpose?
I think he’s a Quicksilver pulled from the multiverse through Agatha’s basement... but neither Wanda’s Pietro nor truly the X-Men’s Pietro, though he’s probably from an “adjacent reality” to the latter. Agatha wanted someone to unbalance Wanda some more and a henchman as well - a multiverse version of Wanda’s brother would fit that bill.