They can even list there and sell Steam keys on their website and not pay any of that to Valve, with the only stipulation that Steam keys cannot be sold for less than on Steam itself.

So basically:

1. You don’t need to publish there
2. But if you do, you can still publish elsewhere
3. And you can sell Steam keys directly with no cut to Valve

You only pay the 30% cut for sales made through Steam.

That’s incredibly reasonable.