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).