The employee should instead give them a warning and educate them. Writing a manifesto is not a capital crime.
The problem is that someone has to bear a cost when opinions like this are made public.
Since the original manifesto author has advocated nothing remotely analogous to that, I would say that your attempted comparison is intellectually dishonest and places you on far more morally shaky ground than him.
Moreover, if "...someone has to bear a cost when opinions like this are made public...", as you say, are you in turn willing to pay the the same cost if public opinion turns against your political views? Any behavior you legitimize to attack your political enemies becomes equally legitimate to use against you when the public mood shifts! You'd have to be bonkers to think this sort of thing is a good idea to normalize.
Maybe you should refer upthread where someone else threw out that comparison, not me? I was replying to them with the theoretical they outlined.
I don't really know how to respond to the rest of your post. Public moods have always shifted, and people's public responses have changed. It's already normalised. People used to be openly racist, now they aren't. Their racism then wasn't correct at the time and incorrect now, it was always wrong. Will my views become equally offensive some day? I don't see why I would assume they will. And if they do, and I don't adjust, maybe I will deserve to be called out for it.
It already happened before in the past and it was directed against the left. Do the names "Joe McCarthy" and the "Red Scare" of the 1950s ring a bell?
https://en.wikipedia.org/wiki/McCarthyism
If Senator McCarthy and the Committee for Unamerican Activities had knocked on your door and condemned you for being a leftist, would you have sheepishly smiled and said (to paraphrase you own words), "Gee, Senator, maybe I do deserve to be called out for it." and quietly accepted the destruction of your life for holding a politically unpopular view? Is that truly what you are saying?
I give up. I can only hope that your movement is discredited and expelled from the left before it does as much damage as McCarthy did before he was discredited.
We also don't hire the KKK. Those of us who run companies or manage people respect our employees more than to have a gay employee pair program with a member of the Westboro Baptist Church. Free speech does not mean immunity from the consequences of your actions.