The issue is not that people can't listen to holocaust denialism or general nazi apologia. Issue is they do and then take action based on that and other people then suffer consequences. Including violent action.

Also, given that Heinlein fiction can be also quoted literally in defense of military dictatorship, it is not like random quote of him was any argument on this topic.

Or any other old big name. The whole "I cut good sounding quote of someone famous or of context" thing is not an argument.