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
I would say it was the adrenaline rush that comes from being part-human that gave him that increased strength for short duration. Same as his power outbursts throughout first season and his fight w Omni man.
I don’t think it was increased strength, I think it was just his normal strength that was no longer being held back by him pulling his punches the way he does on earth against humans. You fight the way you train, it’s called muscle memory, and he’s clearly only developed muscle memory from fighting against people that would implode into red mist if he used his full strength against them.