Epic could just increase the price of the in-app purchases on the Apple platform by the same 30%. Epic gets what they want. Apple gets what they want. Users can learn to use their Mac instead of their iDevice to make the purchase. Bonus of this plan is that if they made it obvious that in-app purchases are 30% more expensive only on the iDevice storefront, the would incentivizing iDevice users to make a non-iDevice their next purchase to avoid this inconvenience.
Since Epic isn't doing that, I believe they recognize that what Apple is doing is not good for developers, and since Epic is a major player they are using that weight to fight the good fight. Apple needs to pay for their store front, sure, but 30% of in app purchases probably rakes in way more cash than it costs them to support in app purchases. A smaller developer may not be able to stay afloat with those margins. The cost to Apple per in-app purchase certainly does not scale with the price of the in-app purchase, so why should their cut? Should be a tiny fixed cost. Ya ya, dOn'T LiKe iT, dOn'T uSe iT... preaching to the choir; no iDevices owned here.