> since that seems to be the bar they are setting for others

this is a very simple exercise in propositional logic (simple enough that i don't need to write a lean proof):

"if you are certain then you should bet"

<=>

"if you're not betting then you're not certain"

so i don't have a "bar" and i'm not saying you shouldn't comment, i'm simply calling you a liar.

this argument clearly does not make sense

It can be summed up as ‘put your money where your mouth is’.

Turnabout is fair play.

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".

"I am 100% sure Microsoft stock will go up 20% tomorrow!"

"i call bs. Why don't you bet your money on it then?"

"well you shouldn't be talking either, you haven't bet money on the stock going down!"