Only he's back. https://news.ycombinator.com/item?id=7834690 We all quit HN at some point in time and those who haven't will do it too. And then we find that there is nothing better. It's a bit like democracy.
The people who quit for real usually don't start with a statement. I think making a big deal out of it is a way of telling oneself why one should quit. Contrast this with someone asking the question "Why did you quit?"and replying with why one did quit.
So forgive me for disagreeing with you but I see it different. Matt was/is an awesome contributor to HN and I definitely noticed him returning, we need more people like him, not less.
And quite a few people who 'quit for real' have stayed away, with or without statements in public. And quite a few 'old timers' have left and have come back (even if not always under their old nick).