The idea is that if we are tolerant of *everything* we also have to tolerate people who will forbid their followers from listening to reason. This is counter to the very values of tolerance.
It’s legit, but there are important points to realise
1. It’s a footnote, not a thesis. It’s an observation that unlimited tolerance is not possible,
2. Popper wasn’t arguing that we should have absolutely no tolerance for intolerant ideas. In fact quite the opposite. He was observing that no matter how tolerant we are there has to be a line somewhere.
Many people see it as a justification for suppressing anything they view as “intolerant” but this is a severe misreading.
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