But I do think some fallacy of the form, "This argument would make UFAI more threatening - therefore UFAI-fearers must endorse it - but the argument is wrong, ha ha!" might have occurred.

I think this is it. However, there are at least a few enthusiasts, even if they are relatively peripheral, who do tend to engage in such indiscriminate argument. Sort of like internet skeptics who confabulate wrong arguments for true skeptical conclusions in the course of comment thread combat that the scientists they are citing would not endorse.