Rational does not need to be the opposite of radical, nor does the parent comment need to imply Stallman did not react rationally. The parent does see his comment as proof of his radicalism which is, as said, a strange statement given that Stallman decries the radical concept of cultural appropriation - a rational statement. Decrying radical statements is not a radical act.

Decrying the concept of "intellectual property" entirely, as he does, is a pretty radical position nowadays, so yes, that comment is proof of a radical position.