There's nothing complex and impossible about removing some "if" statements responsible for code signature enforcement.
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
It doesn't seem like the average Android user is worse-off because of that, security-wise.
But iOS requires that everything be signed by Apple in one form or another. Even debug builds of your own apps you run on your own device from Xcode. IMO, it is absolutely unacceptable to market your devices as general-purpose ones, make the SDK public, but still be an intermediary in app distribution for no good reason whatsoever. I'm surprised the EU is so seemingly patient with Apple's clearly contemptuous conduct.
You think side loading on Android cost Google "nothing" to implement and maintain? No, it costs them engineering resources to support that feature. It's a good feature to support and it's beneficial to users. But it's not free, it doesn't magically insert itself into the Android codebase if they "comment out an `if` statement" as the GP suggested.
Also, Android is gradually adopting many iOS-like permissions and security models. We recently updated our Android apps related to reading and writing to the file system. Why is that? Because the free-for-all they shipped with was heavily abused by developers.
If Apple wants to offer something similar, now, they are going to have a lot of work cut out for them.
You're not thinking this through, it's not a magic button Apple presses. They are going to have to develop a ton of frameworks just to get something like installable APKs.
Apple allows developers to use iCloud and Maps for free. Presumably because you distribute through the App Store. So if they allow for side-loading they're going to have to lock down and split their App Store "services" into a separate framework — hey, sounds familiar? Just like Google Play services.
Separating out all of Apple's authentication layers, paid and cloud services, and ensuring apps can be cleanly distributed without dependencies on those things it not a trivial engineering exercise.
I'm not trying to imply that Apple should not comply with the DMA. I believe they should. I also believe that it would be a seriously complicated thing to extract their App Store services from their developer APIs in such a way that people could develop against a baseline SDK sans Apple services.