Source Engine 2 has been known for quite some time if you follow the scene. L4D3 might have been as well if it is still the prequel that was being talked about a while ago, or if they've changed that now. As far as I remember, they were making a prequel that sort of expanded on the universe they created by being set when the outbreak starts. Though, to be honest I read it a while ago so I don't remember if that's accurate, just what my mind is telling me I remember about it.
Additionally, and not to be a downer, but people shouting "confirmed" are really green to how Valve works. Some text on a screen is not indicative of a game that's definitely going to be released. Valve often drops projects or changes them around entirely. I'm not saying that this isn't FOR SURE never going to happen, because previous news suggest that this game has been in development for quite some time, I'm just being realistic. Valve has had lots of half-life- and portal-related "leaks" in the past that have never seen the light of day. In fact, Half-Life 2 was famously leaked as a playable game while it was in beta, causing Valve to scrap the game and practically start over. The only thing the text really shows is that at one point or another there existed a project called L4D3, but that could be anything. It could be a playground for them to test new Source Engine 2 technology and was never intended to be released as a game, or it could even be an acronym being used to cover up another project entirely (they do this with Half-Life 3 as a joke).
