Oh, an idea being assumed might be objectively true and even demonstrable from logical necessity, but if anyone assumes it, they are to one extent or another disregarding logic altogether, thus making them irrational. It does not matter that it is true or also entirely knowable, and it does not matter if the thing seemed so probable that the person felt comfortable with making an assumption. Subjective persuasion or approval is not objective logical proof, which means one thing by necessity is true in itself (like logical axioms) or in light of some other truth. Other than recognition of logical necessity, which is true independent of all else, there can be no knowledge.
People can still believe or perceive things without having true knowledge because not genuinely knowing logical axioms and other necessary truths does not prevent someone from thinking and experiencing. When they brush up against the fact that they have only assumptions to stand on, or when they feel so persuaded that something is true even though it is not verifiable (or at least they have not logically verified it), they might believe anyway that they really are justified in making an assumption. That some people openly admit they are assuming something while calling their assumption rational only means their delusion is far greater than someone who knows logical axioms but still believes in other things that ultimately contradict them.
Anyone who exists still exists, for instance, even if they have never rationalistically examined anything to find the necessary truths about it. Any non-rationalist just cannot know even this self-evident fact or the logical axioms on which even this depends! To have knowledge, they would need absolute certainty, and to have absolute certainty, they would have to recognize what cannot be false for what it is. There is nothing that cannot be or have been false, because its falsity is literally impossible, other than logical axioms--including the logically necessary existence of an uncaused cause, as it could have been the case that there was neither any beings nor physical substance in existence.
It could not be or have been the case, for instance, that nothing logically follows by necessity from anything else, because then it would follow logically from the nature of reality that nothing follows by logical necessity, and that anything that would have followed logically would be necessity have to be false! That one cannot doubt or reject one's own conscious existence without already existing as a consciousness is likewise epistemologically self-evident, but there is no inherent necessity in one's mind or any other mind existing, only in a mind existing as long as it exists (an application of the logical law of identity) and existing if it perceives anything at all.
No one can be intelligent (rationalistic) regarding a given matter without avoiding assumptions and turning to logic, starting with logical axioms, yet due to ego, fear of abstract truths, social conditioning, philosophical apathy, or any other invalid reason, so many do not. In a world short of perfection, it will always be more likely that a person one is meeting for the first time will be a non-rationalist precisely because shedding or avoiding assumptions takes effort when a person is used to the alternative. Whether the assumptions are passive or active, everyone who is not a rationalist is a slave to assumptions, and there are no safe assumptions. Every assumption makes someone irrational to the extent they assume.