If you view the 90% number as an upper bound, with a few bits' worth of error bars, it doesn't look like such a strong claim. If Szilard and Fermi both agreed that the probability of the bad scenario was 10% or more, then it may well have been dumb luck that Szilard's estimate was higher. Most of the epistemic work would have been in promoting the hypothesis to the 10% "attention level" in the first place.

(Of course, maybe Fermi didn't actually do that work himself, in which case it might be argued that this doesn't really apply; but even if he was anchoring on the fact that others brought it to his attention, that was still the right move.)