It is also suspicious because it means someone has assigned a 0% prior to their being insane or in some sort of Plato's-cave scenario - which is hard to justify.

The minimum possible Baysian prior is a base rate of "my senses are just not picking up reality and/or my memory is catastrophically compromised and/or I cannot process logic right now due to some reason" which while low is never going to be 0%. There are too many known ways for human brains to fall over. 0% priors are unjustifiable.