I think you misunderstood. His mouth was not at "the proof is correct". His mouth was at "you are not really certain that the proof is incorrect". Those two are not the same thing, so flipping the accusation back at him and saying "you should've bet on the proof being correct then" does not actually make any sense.

edit: in other words, if there is anything that he should bet on, it's "the other guy wasn't really certain that the proof is wrong, he is just bloviating". That's a different bet than "the proof is correct".