Just look up where he wrote about this paradox of tolerance? It was in a footnote. (To guard, I'd guess, against people deliberately misinterpreting his words in the main text and going "Ha, look at this doctrinaire free-speech absolutist." I've read the book that was in.)
Why does where it was written matter? Saying that it being written in the footnotes invalidates the argument is the logical fallacy of poisoning the well.
You would actually need to refute the argument directly for your assertion to have any weight to it.
- People make a big deal about renowned philosopher Karl Popper warning us of the danger and incoherence of tolerance towards free speech. I think this misuses his rep.
- The one-paragraph quote on the Wikipedia page linked above was Popper's full writing on this. You don't need to look further for it.
Others were already addressing the object-level arguments.
