"First they came for the <relatively unobjectionable group but which I wasn't a part of> so I remained silent".
https://en.wikipedia.org/wiki/First_they_came_...
First they came for the Communists
Then they came for the Socialists
Then they came for the trade unionists
There is nothing inherently wrong with any of the groups, you may or may not agree with any of them but wanting Communist ideals isn't inherently bad. Capitalists normally use the phrase that it all sounds good until it meets humans. Same with the socialists, who all they want is socially derived benefits from community owned services, and trade unionists just realise that the only way to gather any strength is through numbers. None of these ideals are bad.
Saying things like "first they came for the neo-Nazis..." completely misses the point.
I think the folks using this form understand the history of the saying but are deliberately changing it to show they are OK with the bans as long as they are limited to neo-nazis and the like.
Once it moves past these neo-nazi recruitment forums we can start worrying about whether or not it is a slippery slope.
If not then you should defend them in the same way that you would want to be defended. If you object to basement dwelling troll groups then the saying will also have no impact on them.
I don't really care if the saying will have an impact on them. As far as I'm concerned they're unreachable -- at least I have zero chance of convincing them of anything, especially if I can only talk with them in a hate forum.