I actually think this was handled well.
I'm normally against taking punitive action against employees for old social media content. People make mistakes and they should be allowed to grow and improve without being held permanently accountable for their past selves. However, this isn't like James Gunn being fired for old jokes with bad taste. This person was Google's head of diversity and he posted serious, blatantly-antisemitic arguments. That post would compromise his credibility with subordinates - not just as a person, but as a leader with a focus on diversity.
I like how Google reassigned him instead of firing him. It's an old post and he has hopefully changed for the better since then. He's no longer fit for his previous role, but there are probably many other roles he can still be effective in. I don't want to see someone crushed for past mistakes if they've learned from them.
IMO, this sets a good precedent for how to handle issues like this in the future. Hopefully other companies are taking notes.