Apple's philosophy is that new APIs need some time to stabilize before they can be baked-in as a commitment to third-party developers.
So new APIs are almost always first-party only. Apple designs the API and becomes the first consumer of it. This experience of dogfooding their own APIs lets them iterate and learn without breaking compatibility with third-party developers consuming the API.
Only after an API has been hardened in this way does it become eligible for third-party consumption, where Apple can promise to document and support those APIs publicly.
It makes sense then, that if the DMA mandates equal access to new APIs for third-parties, then Apple will just disable new first-party APIs in the region until they've gotten their bake-in period elsewhere in the world. Sorry, EU!