That seems a-historical. The way you get the real nazis is by not acting soon enough when they target/vilify/'other' marginalized groups, or attempting to get a foothold in political institutions.

It is ahistorical. Germany outlawed Nazi ideology and went 80 years without renazifying; the danger of such was very real after WWII. Marcuse, a German Jew, was considering the real, immediate, terrifying problem of how to prevent another Holocaust.

You're cherry picking.

Look at Joseph McCarthy and see a counter-example. He used the exact same logic: that to suppress Soviet-style totalitarian government in the US, we had to be intolerant of all far left views.

It destroyed many peoples' lives, most of who were not at all sympathetic to Soviet-style communism.

Similar to your example, we could also argue that McCarthy was indeed effective at preventing Soviet-style communism in America (since the US still has free speech and there has not been another holocaust). But in both cases we don't have convincing evidence that suppressing speech was actually helpful.

Worth adding on that McCarthy knew exactly what he was doing and didn't care. It raised his profile. (Source: master of the senate, caro)

Those who validate dirty pool in pursuit of the greater good will always be enabling people like that.

Being moderate is a delicate balance where foes seek power in whatever way possible. With this said I get quite concerned when someone says it's their freedom to plot the end of my existence.

Germany still has plenty of open Nazis for all their efforts at censorship. Something like the infamous march in Charlottesville was breaking news when it happened precisely because it's so unusual, but much larger neo-Nazi marches happen in Germany on a regular basis.