> I'd assume they'd love to rake in engine licensing as soon as possible

Source does rely on several other technologies (i.e. Miles, Blink, Havok) that you must license from their respective license holders, however Source is free to use if you're distributing on Steam and you only need to license the Source Engine for distribution off Steam or if you choose to distribute the SDK's tools with your game.

So with that in mind, I doubt the rely much on licensing fees as a source of revenue.