Because "no politics at work" isn't neutral, it means supporting the status quo by way of defining what counts as "politics". Is calling someone by their preferred pronouns, or wearing a mask during the pandemic politics?
You can hate that argument, people can still accuse you of it and they might be right.
And, “there’s nothing we can do” isn’t true for the United States citizens who can vote the most powerful economy/military in the world.
I don’t think we should be making rudeness illegal. On the other hand, professors absolutely shouldn’t be able to sue their institution for punishing them for it. There’s a middle ground
The question in this case is "should it be illegal to discipline an employee for deliberately misgendering a student?" At least one federal circuit court found that it is illegal for the university to discipline a professor for deliberately misgendering a student multiple times.
This is all to say, being deliberately misgendered by a coworker is a real problem. A private employer would not be required by law to fire or discipline an employee who was doing the same thing as this professor. So for the employer, deciding whether or not to discipline an employee who deliberately misgenders a coworker is "a political issue"