Are we pretending publishers not bothering putting their games on every storefront is the same as paying publishers to not put those games on competing storefronts?
Steam was literally forced on those who owned a physical copy of Half-Life and wanted to play it. The dominant position has nothing to do with the service offered by Steam. It was dominant when it barely had any features. GOG competing with it on features and in fact offering the bonus of DRM-free games hasn't improved its market share of about 0.5%.
Epic: then we have no interest in having you on our service
If anything, the example you brought up proves the opposite. Darq is on Epic and its developer even took money from Epic to make it free, so there is no grudge even past the dev's publicity stunt.
Their attempt to strong arm an exclusivity deal failed and at some point they relented and put the game on their store.
If they had just hosted in on their service at the same time in launched on Steam it would have been better for their customers and more profitable for Epic. But they are more concerned about trying to force exclusives than do what is better for their customers, even if it loses them money.