My understanding is that they can negotiate that Steam take a lower cut of sales if they don't make use of Steam's DRM or other platform features.

Steam is convenient but monopolies aren't good either. In a perfect world all the platforms would provide proper API's so the community could make nicely integrated open source launchers that consolidate all of our games like GOG Galaxy tried to be. Given how finicky GOG is, that unfortunately seems to not be a world we're in yet or maybe ever will be.