Yes, I know what you mean ZNOP, but then they retconned it to be someone else. At least, that is what I heard.
Nanomachines. Evertything about Xorn was explained by nanomachines with Kick used to amplify his powers far greater then they actually were. Even with the retcon saying he wasnt Magneto, those two things are enough to explain that
What Medusa thinks in the middle of the Battlefield where the X-men could be hearing:
"Why didn't they just tell us, Iso ? we would have found a solution. It didn't have to become...This"
What Medusa thinks when the battle is over and she is back and safe in New Atillan:
"Our species' ability to transform through Terrigenesis does not outweigh the lives of even a single mutant"
That's why the retcon of xorn being a magneto imposter had to happen having magneto commit genocide against half the human population of New York City didn't look good at all for magneto as a character...also if I recall there was a backlash to what Morrison did with that story didn't many fans like hate that story? Also I hated how Morrison had xorneto kill Jean it was so anticlimatic.