Valve just doesn’t allow cheaper prices from other storefronts if it’s a steam key being sold, where valve is the one footing the bill for the server costs. There are games for sale on epic all the time that are better deals than what’s on steam. But when you buy a game on epic, you’re using epics servers/bandwidth.
Which is all that would happen if third parties decide they don’t like the terms that valve and them agreed to.