Kinda wish he did to save face.
Breaking a promise made in public, even if he won he would be even more ridiculed from breaking the promise he made to someone he thought a weakling.
By complying with his promise, he would've saved faced in this fight (even if he lost, the outcome would've been "well he wasn't as weak as your expected, it's your loss on your perception abilities", plus he clearly isn't someone who could've made it to the top, would've been best to stop here).
And further in, if our MC showed to be way stronger than everyone thought, this antagonist's loss would've wiped and forgotten.
Welp. That's how I wished it'd be, of course, the shortest path is the simplest as we see it every time...