I sure know that in my brief considerations of the matter, the paths of thought have consisted of a lot of 'Well if this is true, then that is almost certainly true' and lots of meta- and meta-meta-uncertainty. E.g. 'Well, if X and Y (meta)epistemological/(meta)metaphysical propositions hold, then it seems like P(Simulation) is zero/is a half/is one. But I'm not sure if I'm overlooking some class of cases that weaken the implication, so maybe it merely seems like P(Simulation) is that high under those propositions, but actually it's not'.

That immediately leads to lots of cases arising as X, Y, and 'seems like' are variously true or false or not applicable. So it wouldn't surprise me at all if uncertainty over arguments gave rise to the middling estimates.

I also wouldn't buy the latter. It wouldn't have occurred to me as an explanation.

Given he's the expert on the simulation argument, I was pretty disappointed that there wasn't an estimate by BOSTROM for it.