It's the first time they acknowledge Half Life 3 though. Their only previous continuation of Episode 2 was Episode 3, pretty much.
I think the most important question is: Why now? During the hype for SteamOS and the controller. GabeN pls . But I bet he reads Neogaf daily, watching us squirm while he polishes his knife collection. (When waiting in the DotA2 lobby of course)