At this point, I feel like Apple is just playing chicken with the EU. If this isn't "gatekeeping" (ie, the thing the DMA is supposed to prevent), then I'm not sure what would be.
IIRC Epic has been caught several times in the past few years breaking store rules, like sidestepping the platform payment system and misrepresenting data collection attestation. I don’t think it’s anti-competitive to decline to sign a vendor’s apps. I mean, I like Epic’s developer platform, but it sounds like their lawyers are high if they think EU courts will force another company to do business with them, especially if management has been intentionally abusing the system.