Why do you think this claim is "wildly" speculative as opposed to merely speculative?
We have two possible options here, it's pretty obvious which is the more likely one.
It is pretty ridiculous to suggest that nobody has ever been caught cheating in overwatch pro games.
Again, you are missing the point, just because something is "likely" to happen doesn't mean it did happen.
What you are basically asking is that we should provide a "negative proof", imagine me going through all the pro matches to prove my point that it did not happen (going in this extreme) when you can just show me a proof that it did happen.