It’s extremely complex. I’m not debating whether they should comply - they should. But it’s gonna cost them years of engineering effort, and maintenance far into the future. See, for example, BrowserEngineKit
https://developer.apple.com/documentation/browserenginekit
They needed to engineer, maintain, document and support a whole class of APIs so that third parties can create their own competitive browser engines (that offer JIT, etc) while still maintaining iOS sandbox security. There are going to be hundreds of frameworks, thousands of APIs, that will need to come to ensure compliance with the DMA