You haven’t articulated what your position on free speech really is, but from what you imply about it, I think you are misguided.
One can use speech to articulate hateful viewpoints, but one can equally well use speech to articulate the rejection of hateful viewpoints. Historically, the latter have been successful.
One can use speech to recruit and radicalize, but one can equally well use speech to dissuade and moderate. Historically, also, the latter is dominant.
One can use speech to allude to dark thoughts, but one can equally well use speech to bring up those thoughts directly so that the alluder is forced to be explicit, or to repudiate them. Again, the latter is the more successful strategy.
In a society with an excess of speech, everything is better.
If radical white nationalism worries you, you want more speech, not less. Bad ideas shouldn’t be allowed to fester secretly, unchallenged intellectually, but merely shouted down. Evidence, reason, and rhetoric are, when applied liberally, toxic to most forms of radicalism.
Let the alt-right try to use free speech to accomplish something! Unless they have good points, it will backfire.
If, on the other hand, you want to strengthen a left-leaning in-group, then yes, anything the right says about free speech is a threat. Listening to others’ arguments makes it harder to demonize them, and nothing rallies allies to your side like the threat of a demon.