Well, in that post by Xixidu, there is a quote by Mitchell Porter (that is approved by Eliezer) that, combined with the [reddit post] I have linked earlier, seems he was not able to provide a proof that no variation of basilisk would ever work given that there are more than one possible decision theory, including some exotic and obscure ones that are not yet invented (but who knows what will be invented in the future). Eliezer seems to think that humans minds are unable to actually rigorously follow such a decision theory strictly enough that would be required for such a concept to work. But the human ability is such a vague concept, it is not clear how one can give a formal proof.

However, it seems to me that an inability to provide a formal proof seems to be an unlikely reason to freak out. What (I guess) has happened was that this inability to provide a proof, combined with that unnamed SIAI person's nightmares (I would guess that Eliezer knows all SIAI people personally) and the fear of the aforementioned potential PR disaster might have resulted into the feeling of losing control of a situation and made him panic, thus resulting into that nervous and angry post, emphasizing the danger and need to protect some people (and leaving out cult PR reasons). This is my personal guess, I do not guarantee that it is correct.

Is an inability to actually deny a thing equivalent to a belief that negation of that belief has a positive probability? Well, logically they are somewhat similar, but these two ways to express similar ideas certainly have different connotations and leave very different impressions in the listener's mind what was the person's actual degree of belief.

(I must add that I personally do not like speculating about another person's motivations why he did what he did when I actually have no way of knowing them)