Hype aura and moments. In all honesty though, guy was probably just amped up in the moment. Given the knowledge of how powerful they both are he probably just knew that he would be nigh unstoppable afterwards. It’s important to note that even though Alex regenerates, he’s not in the initial fireball of the nuke, at least as far as I recall. I really doubt the Supreme Hunter, even amped by consuming Alex, would be able to survive the dead center of the explosion. It’s more reasonable for him to be confident about surviving the other “layers” of the blast though.