Currently the coop campaign is designed to be "server less".
Meaning it does not need any of ZK infrastructure working at all.
Steam is used to arrange following things:
1) find friend and assign to game
2) transfer initial game state (campaign state)
3) provide reliable networking possibly with use of steam relay servers (these days with NATs and other stuff it is actually not so easy to find "open port", we use steam to provide two sided holepunching and backup relay if needed). Your spring game actually connects to "localhost" while we relay packets via steam system.
