> It's not free money on the ground, it's a bet on the outcome of some event.

If you have convincing enough evidence for it, a bet is more likely than something that looks like money on the ground. Imagine betting the sun will rise tomorrow.

> You're presupposing that the only possible rational reason they might have to not bet might be that they are not confident but that's obviously not the case.

They didn't say it's the only reason, just that refusing to bet weakens their claim of confidence. Plausible but unconfirmed reasons are why it's only a weakening rather than disproof, but it's still a weakening.