These exclusives have nothing to do with the cut. Its entirely to do with the exclusivity payments.
Epic store currently makes a loss for Epic due to those payments from Tim Sweeney's own admission. Which means its a lot of money that epic is handing out for essentially free, there have been multiple accounts of companys saying even if they sold zero copies on the epic store they would be making more than their predicted sales.
So even if Valve cuts their take I don't think it would change the situation.