Well reading between the lines, and this is just my speculation, it is obvious that the mod hub, is not being powered by Steamworks, Greg hasn't come out and said this per se, but he's said quit a few things that make this a logical conclusion.
So 2K is using some other method/server to host the mod hub, now as far as my research has shown 2K isn't big into running game servers themselves, so I'm going to go out on a limb and speculate that perhaps Gamspy might even be the host for the mod hub.
Why would I think that, well I is obvious that MP was developed very late in the dev cycle for Civ5, or they would have been happy to push details alot sooner than they have been. We still don't know alot of details even now. So what I am thinking is Steam(works) was a decision made late in developement as well. And if that was true, was was planned for MP before Steam? Hmmmm, Gamespy perhaps like every other Civ game since civ3.....
And perhaps because of this 2K already had a contract with Gamespy and wants to get something for their money....and hosting mods is definitely a better role for GS than getting it's fingers into the p2p code!
So I'm betting that is the back end for the mod hub that is coded into the game interface and has no direct links to Steam at all. And any web server functions are likely handled by GS as well for the in game web browser.
But on the other hand I could be wrong, time and netstat will tell
CS