I'm not asking where to get it -- but can anyone confirm this as truth? If so, can someone explain to me how that's possible?
It depends. It could happen any number of ways.
For instance. The source to Half-Life 2 (IIRC one of the Quakes also fell victim in a similar way) was stolen when someone successfully exploited a flaw in Outlook in order to install a Trojan into Gabe's computer. From there, the person gained access back to the main host server and other selected clients. During this time, Valve kept telling the fans that the game was nearing completion and the release date was X. That every thing was kosher. In essence, Valve lied to the community when the game was nowhere near completion. When the release date came and gone. The person downloaded and released most of the source code up until that point to show the community that the game was nowhere near GOLD. ANON. From what I can tell, gaining access to the CVS server in some fashion seems to be the most common way.
This happened to several games, but I don't recall which ones off the top of my head. A developer and a team had a bit of a falling out over some thing or another. In a fit of rage, the developer copied the code, left the team, and released it to the public.
Someone on the "inside" but not generally a developer gets a hold of the game and releases. More common with the game itself but not the source code. Generally, these are people who work as game testers, QC, or someone else in the chain of the process. For example, Microsoft pre-release OSes make their way to the open net because it's actually pretty easy to become a member of the MSDN and sign up for BETA testing new OSes as they come out.
Sometimes, the developer releases the source code in some fashion. Willingly or unwillingly. Half-Life, Quake, and Unreal fall into this category to some degree.
I'm sure there are others, but I believe those are the biggies.