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saying

"they're aware that in an inflationary economy an "interest-free loan" isn't merely a opportunity-cost for the lender, it's a concrete cost.

Is merely repeating a popular myth. The existence of an inflationary economy is a purely rhetorical invention. As is the idea of a concrete cost. Opportunity costs have the same story. They're just ways of understanding the world but they are not objectively true.

So while that user may believe the myth that you articulated, that myth is not itself true.