It isn't even evidence until you include a premise about the likelihood of y, which we agree is the implied premise.

This is obviously going to be the case when trying to convince an individual of something. The beliefs (crackpot or otherwise) of the target audience are always going to be relevant to persuasively. As a comment directed in part to the wider lesswrong audience the assumed premises will be different.

Try "Robin Hanson thinks we should cut health care spending 50%, therefore he is less likely to be right about fertility rate."

If I were a reader who thought Robin's position on health care was as implausible as belief in magic and thought that making claims about the fertility was similar to AI strategy then I would take this seriously. As it stands the analogy is completely irrelevant.