There is already a deal between Valve and Facepunch. I don't know all the terms but you will need to publish your game to Steam (not exclusively).
https://sbox.game/dev/doc/systems/game-exporting/ (bottom of page)
https://sbox.game/dev/doc/systems/game-exporting/ (bottom of page)
> You can export your game, but you shouldn't distribute your exported game just yet.
That doesn't sound good, maybe they realized after the fact that they haven't actually worked things out with Valve?