However what it does confirm is that it likely isn't in the pipeline anytime soon.
How does this confirm anything other than a writer who worked for Valve left the company? We don't even know if he was working on Half Life 3, and even if he was, the story is one of the first things that is decided upon when developing any game (assuming there is a story). I'm sure Valve employs at least half a dozen other very talented writers, some who may have even had a hand in the other Half Life games as well. Would any of them quitting mean anything in regards to Half Life 3? No, it wouldn't.
People need to calm down. It's like anytime there is "news" regarding anything Half Life people want to make it into something that it isn't just so they can go around and spout "Confirmed!!!" or "Cancelled!"
It will come out and it has been under development, I would bet money on that. It sucks that it has been nearly 9-years since we got episode 2, but I'd rather wait a decade for another ground breaking title than a COD clone with aliens.