He’s what? 18? Basically nothing in viltrumite timeframe.
Literally gets dominated by Lucan using one finger.
Then he gets his ass beat by Thula, who has hundreds of years of experience minimum, mind you viltrumites get stronger with age, aaand somehow a pep talk allows him to almost kill her? What? Mind you, in the comics this Thula threw down with Battle Beast and walked away alive
Invincible strength levels have always been a little iffy (this can apply to comics in general). I see it as Mark being enraged and not holding back makes him stronger. I think there could also be a Goku effect.
Also, this is a spoiler, but >!I think someone mentioned before that human-viltrumite hybrids can actually be stronger than pure viltrumites!<