one meaning of the word “rational” is “has authority, is Right". even “True”, sometimes. this is a bad meaning, but a standard English meaning nonetheless.
basically ppl are justificationists and use “rational” to mean what’s good in that epistemology. and the basic point of justificationism is: if an idea doesn’t have authority, why would anyone (rationally) accept it? the "rationally" excludes reasons like arbitrarily due to taste, or chosen by self-interested bias.
what’s good in their epistemology is authority and being Right and having the Truth.
for this approach to reason, an idea can be rational. saying an idea is rational is claiming it is True, or at least has Authority.
a better approach is to see reason as a process. the point is to think about ideas in a way that can make progress. that means that if a mistake is made, the process can find out and correct it. error correcting approaches are rational, and ones which prevent error correction are irrational.
so, the standard idea most people have is: knowledge = JTB (justified, true belief)
meaning you should Believe ideas which are True, but you only get credit if you believe them due to authority (justification), NOT for any other reason (b/c if they don’t have authority but turn out true, why'd you believe it? that wasn't a rational belief, you didn't know it was true, you just got lucky that it happened to be true)