The rumor had been for a while that DOTA 2 was going to be their testbed for Source 2, like TF2/L4D2/Portal 2 had been for a lot of things for a long time and DOTA has been since. Cloth and stuff was already in it.
From what I've cobbled together in my head over the years, it seems like Source 2 is going to be a lot more about the content creation and UI than something major to the overall base. They already built in all sorts of stuff to get rid of the old map limitations and stuff but all the old games don't support it. Compare L4D2 and Portal 2's map loading to HL2's and such. Especially now with how other engines let you pop-in and out*, while Source still had you compiling maps with whatever you want to call that "tool" for making things.
Titanfall always impresses me a bit when I stop and go, wait, this is on Source. There's probably no way you could have made maps like that on it five years ago, no matter how much Respawn altered. They couldn't have picked it if a lot of the stuff they wanted to do hadn't already been dumped into Source just nobody but Valve was using the engine. Alien Swarm had a bunch of dynamic lighting stuff Valve never used too.
Wonder if they'll do anything to CS:GO, or if its takeover of the competitive community makes it hands-off.
*Fuck, even Far Cry 2's map editor let you do this.
