Why couldn't he have, like the other LWers apparently did, considered the possibility and then (erroneously, according to your epistemological state) rejected it?

My experience reading Eliezer's writings is that he's very smart and perceptive. I find it implausible that somebody so smart and perceptive could miss something for which there is (in my view) so much evidence for if he had engaged in such consideration. So I think that what you suggest could be the case, but I find is quite unlikely.