They would be classed as irrational based on the belief that they do not, in fact, have ‘rational reasons’ for their decision. If that belief is false then it is false—just like any other.
I read this statement as an arrogant assumption that a sufficiently rational person could expect to correctly judge the probability of cryonics working with enough certainty that disagreeing with them would be “just like” disagreeing with reality.
I read “that belief” as referring to “the belief that they do not, in fact, have ‘rational reasons’ for their decision”. “Just like any other”, then, probably refers to the fact that many rational beliefs turn out to be incorrect (rather, frequently the optimally epistemically rational degree of credence for a proposition is further from the mark than a degree of credence selected by an alternate method, or is somewhat high despite the belief actually being incorrect, though on average rational degrees of belief are more accurate).
What if someone has rational reasons for rejecting a belief such as cryonics, but is deliberately using Dark Art rhetoric to talk more convincingly about that belief by associating it with low-status people? You’d class them as irrational when you should class them as unethical.
In this case, it might be (epistemically) correct to class them as irrational (with some probability, etc.), given the information you have about them.
Similarly, if someone draws a card at random from a standard 52-card deck, your degree of credence that it is the seven of diamonds should be 1⁄52 - it wouldn’t be correct to be more confident than that, even if in actuality it IS the seven of diamonds, as this is information you do not have access to.
(ETA: I’m speaking abstractly here—making no comment on rational beliefs about cryonics.)
I read “that belief” as referring to “the belief that they do not, in fact, have ‘rational reasons’ for their decision”. “Just like any other”, then, probably refers to the fact that many rational beliefs turn out to be incorrect (rather, frequently the optimally epistemically rational degree of credence for a proposition is further from the mark than a degree of credence selected by an alternate method, or is somewhat high despite the belief actually being incorrect, though on average rational degrees of belief are more accurate).
In other words, this is not entirely correct:
In this case, it might be (epistemically) correct to class them as irrational (with some probability, etc.), given the information you have about them.
Similarly, if someone draws a card at random from a standard 52-card deck, your degree of credence that it is the seven of diamonds should be 1⁄52 - it wouldn’t be correct to be more confident than that, even if in actuality it IS the seven of diamonds, as this is information you do not have access to.
(ETA: I’m speaking abstractly here—making no comment on rational beliefs about cryonics.)