Indeed. Never assume that anything you write at work on a work computer won't be seen by your manager or used against you. Even if it's supposedly confidential correspondence with HR or protected labour organisation, and theoretically illegal for them to access. Even if it's personal info under GDPR.

And especially not if you post it to a huge internal mailing list.

(I'm also curious as to what the overlap is between believers in "at-will firing is great and there should be no notice requirements on employers" and "Damore should have had far more process before being fired")