If you completely ignored how he had been attacking up to that point and just saw that panel in isolation, I could see being confused. However, he'd been flicking his fingers up to that point to attack, Todoroki thought he was "out of ammo" since he had busted all of his fingers by flicking them. But then he sticks his thumb into cheek, stretches it out and... bites it? No.MHA fights are pretty easy to follow, but characters sometimes do small attacks that are not always super clear exactly how it was done.
This for example:
In the manga I thought he bit his thumb. Nowhere in that panel does it tell me that he's gonna use his mouth to flick it.