(i.e., a manager can and should defend an employee against sexist behavior, and such defense isn't a priori sexist behavior itself even though the defensive behavior wouldn't have ever taken place if the employee weren't of their particular gender, which would of course make it otherwise look sexist.)

Can you quote one example?

A good friend of mine just get out of an impossible job position where she was under constant oppression of her boss and one of her subordinate (that incidentally was a friend of her boss)...

She started documenting everything so that when thing escalated she had proofs to show the big boss and guess what the big boss sided with her. But she left anyway for a better work environement.

But guess what, this story involved only women (boss, big boss, subordinate). So that's why I think that no matter which "type" of discrimination you met at your job place (sexual, racial, physical, age, etc...) the common response is "document you experience". Cause you'll need this to sound credible.

PS: For people who can read french, I'll recommend reading the "comme convenu" story of the illustrator Laurel. She had documented the bad experience her family has gone trough in the form of a web comic book. They choose to co-found a startup in California but were basically hostage of a startup CEO that hold them using financial and visa menace. (The story is not yet finished) https://commeconvenu.com