It's kind of weird, there was a time when Moss was talked about as a quality, rising-career type guy, with media talk suggesting he might be a head coach or defensive-coordinator candidate. Didn't he get several D-coordinator interviews some years back?
Think the tweet fits with his persona: tough, gruff, take-no-guff hard guy, with no flattery or smooth talk, just a snarly straight-shooter.
The persona doesn't really speak to the ability to coach and teach; based on his press conferences, he seemed to be a pretty poor communicator, and it's somewhat hard for me to imagine him having been a very good teacher as a result.
For him, I assume he knew that with MM gone, and with his linebacker crowd being so unproductive, he probably knew he was going to be cleared out after the season anyway. Not sure getting fired bothers him much.