I agree with you that it is possible your flat-earther may have been dishonest, but I think the social-psychological dynamics that
Tali Shalot mentions in the following video (5 minutes) unfortunately have to be considered in the evaluation of our harboring expectations of another person's epistemic culpability here:
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So, my earlier affirmation about the psychological presence of "locus of trust" plays a part in the extent/degree to which any one person is willing to adapt and/or adopt new information into his existing view of the world,
even if that new information is in essence updated and completely accurate. Sometimes the locus of trust catalyzes a delusional state in a person; sometimes, though, that same relation with a trusted source simply causes a person to be hesitant to accept outside information, even though he or she intuits or understands that it is 'technically' correct.
Some of what I'm saying here should be already familiar to both of us since we both read (and trust) respective sources on neuro-science and psychology, or even evolutionary psychology. It also plays into how you prefer to listen to someone like
Robert Sapolsky but I prefer to listen to someone like
Malcolm A. Jeeves.