This isn't a really a fair reading of the student's letter. It's clear that she was puzzled by the apparent (and in fact actual) contradiction between the erroneous paragraph and the rest of the book. She just asked Feynman for an explanation of what the paragraph meant. It does seem reasonable to give Feynman the benefit of the doubt and assume that there might possibly be a non-erroneous interpretation of the paragraph in question.
I think Feynman was getting on a favorite hobby horse about not trusting authority and reading the letter a little uncharitably.
I think Feynman was getting on a favorite hobby horse about not trusting authority and reading the letter a little uncharitably.