That matches what I've heard, but we don't know what his behavior was like in the office. I think GP's point is that this may have just been (one hell of a) final straw.
His previous behaviour is as irrelevant as the content of his post: he was given an expectation of safety to express his opinion and that trust was abused by multiple parties.
Is it? If he has a pattern of making statements deemed by HR demeaning to women (or whatever the problem was) as was told "don't keep doing that or else you're fired" this could easily be seen as violating that agreement.
