Absence of evidence is no evidence of absence. You could put the Bayesian prior to be extremely low, but a zero chance would not make you a Bayesian anymore, it would make you a believer in that something is simply not possible (and no amount of scientific evidence would update your priors. It really is scientifically a mistake to claim: There exist no black swans. To proof that, one would have to observe all of existence. Now... should you worry about black swans, when all you see is white swans? Depends on you and the amount of risk managing. But that poster claimed all of priming is non-scientific, when we have clear replicated proof.
Suppose I run a fake investment company that pretends to double your money, but actually just steals it. Suppose you find out all my claims are lies, and demand your money back. How would you react if I said "The absence of evidence is not the evidence of absence. The prior probability of my investment strategy working is nonzero. Now give me more money."
Just pointing out that priors can't be zero isn't a principled stance, it's Pascal's mugging. You can use it to justify literally anything.
