The argument boils down to "it's impossible to know if the opposing side is truly being intolerant." You say it is impossible. I say it isn't.

Close, but not quite. That situation does, indeed, arise, but what I am arguing for is a philosophical model that provides valid results even when applied by my worst enemy.

While we can certainly come up with any number of subjective characteristics distinguishing you from them, there is no objective distinction between your brand of intolerance and theirs. As the subjects of your intolerance, they have just as much a claim to declare you fascist as you have to declare them. The tragedy of Popper's paradox is that it absolutely requires, but does not give any guidance in determining who is the good guy and who is the baddie. In the form commonly presented, It just tells you it is a moral imperative to oppress your enemies. That's a big fucking problem when history eventually determines you were on the wrong side of the issue.

The free speech absolutist does not have this problem. He recognizes that he does not agree with his opponent, but he understands he is not empowered to silence his opponent. This is true regardless of who thinks themselves the good guy.

Popper's paradox calls for fascist reactions to fascism. Popper's paradox calls for the echo chambers and deepens the divisiveness that underpins so many of our societal problems today.