u/Cuxrie

Mark already thought his dad was executed and told everyone as such. He could have introduced the returning Omni-Man as a good variant, and since Angstrom is still alive, it's not implausible. The world already knows about the existence of variants from the evil Invincibles, so if Omni-Man pretended to be a good variant of Nolan, he would not have to deal with the consequences of his previous actions.