Your interpretation requires distorting his statements to mean something that they are not meant to mean. You're inserting exclusivity where none exist and exaggerating the use of specific words to mean more general things.

instead of writing a text book, those professors should put their content online for free

You're conflating textbooks with all possible ways of selling or distributing knowledge, which Seth is not. He is talking about textbooks specifically, nothing else. There is no ambiguity there, the whole post is about textbooks. He does not say to publish textbooks for free, but to publish something other than textbooks. He does not say that all books are bad, but that textbooks are bad. He does not say that people should not be allowed to profit from their work, but that doing so by selling textbooks is bad.

Your accusation of hypocricy only flies if you draw general conclusions from his specific statements.

you also agree that he uses the blog only as a marketing channel

I did not say that and do not believe that. You're being dishonest and distorting my words by inserting exclusivity where none exists. Please stop.