I had thought they'd use GfWlive. But of the two, Steamworks makes the most sense.
Impulse wasn't an option because Impulse Reactor does not presently have an in-game overlay for doing MP matchmaking.
In Demigod, GPG had to code all the lobby stuff. Impulse Reactor only provided account and game lists in a DB the game then access and display (and does) but that's extra work.
The Impulse overlay wasn't supposed to be released until Elemental originally but a very very basic version was put out with Demigod so that players could chat. Some o f the beta testers can provide more info on this as they were there at the time.
Anyway, as a practical matter, SupCom 2 had either Games for Windows Live or Steamworks. I wish Valve could make Steamworks work without their game client (Impulse Reactor, as others have pointed out, doesn't require Impulse to be installed).