Yes, you have made a convincing argument, I think, that given that a proposition does not involve negation, as in the alien's question, that it is more likely to be false than true. (At least, if you have a prior for being presented with questions that penalize complexity. The sizes of the spaces of true and false propositions, however, are the same countable infinity.) (Sometimes I see claims in isolation, and so miss that a slightly modified claim is more correct and still supports the same larger claim.)

ETA: We should also note the absence of any disjunctions. It is also true that Alicorn is sitting on a blue couch or a red couch. (Well, maybe not, some time has passed since she reported sitting on a blue couch. But that's not the point.)

This effect may be screened off if, for example, you have a prior that the aliens first choose whether the answer should be yes or no, and then choose a question to match the answer.