Steam server hosting is different from TNet. It's meant for a very small number of players (think no more than 4-player coop games), and it's done by basically sending messages through Steam's API that will be forwarded to other clients. It's nothing more than a mirror to bounce your packets against.
TNet is not related to that in any way, nor does it need Steam. That said, I've done Steam integration in the past with Windward, and most recently with Sightseer, if you can call it "integration". I basically host a local TNet server as normal then use Steam's API to set the user's public status to be playing the game, with a public IP address that their friends can then join by right-clicking the friend's name in their friends list. Doing so effectively sends a notification to the Steam's API that you can then catch and initiate the connection within your game.