I'd actually be curious to know a) what the relevant policies were in this environment concerning such language, b) whether Mosley had been thoroughly briefed on such policies, and c) exactly what dialogue led to Mosley receiving the call from HR.
To wit:
Let's say the TrekBBS has a policy saying you can't say "foo" and I'm aware of said policy. Let's say someone in a thread says "My grandmother once said her neighbors were a bunch of foos."
Now, technically this is a violation of the policy, even though the poster is presumably quoting a third-party and may even be opposed to the use of the word.
Perhaps the use of "foo" in this context bothers me because I feel triggered. Perhaps I don't have feelings one way or another about the word itself, but I am aware of the policy and I'm trying to be conscientious.
Perhaps I report the post, but in the context of "I don't know whether this is really an issue, and I'm not trying to make trouble, but I know there's a policy about saying "foo" on the board, and this poster said "foo"."
All of which is to say, as others have noted, there's a lot we don't know about what transpired, and one of the major missing pieces is what the motivation was.
That aside, "Please don't say "foo" when you're at the workplace." in broad strokes seems like a fairly harmless slap on the wrist to me, but I can see circumstances in which, depending on the value of "foo", I might take umbrage to being asked not to say it myself. Do I think I'd take enough umbrage to actually quit a job in response though? I'd probably feel my work environment was less harmonious than I previously suspected, but I don't know that I'd quit over it.