It's entirely possible that to enable compliance with this will require explicit compromises to the security of iMessage, for example by requiring key exchange with startup messaging providers. The other rules seem to prohibit Apple from describing the risks involved in such a compromise.
> * Use existing hardware and software features without competitive prejudice. E.g. NFC
This appears to say that a malicious app can present UX elements that were previously limited to the OS. Read liberally, that would mean that e.g. any app could now get biometric data by presenting a fake privilege escalation screen (e.g. FaceID/TouchID) and then capturing the results from the Secure Enclave. Is this something people really want?
> * Not preference their services. This includes CTAs in settings to encourage users to subscribe to Gatekeeper services, and ranking their own services above others in selection and advertising portals
This will likely make it harder for users to find safe/private services to use. If every offering can find its way into the default browser/App Store/etc. settings page in the OS, scam services will appear to be endorsed and therefore legitimate.
Edit: Limitation of interop is still possible, the EU is just deciding to move decision making from California engineers to Brussels attorneys. From the Act:
> The gatekeeper shall not be prevented from taking strictly necessary and proportionate measures to ensure that interoperability does not compromise the integrity of the operating system, virtual assistant, hardware or software features provided by the gatekeeper, provided that such measures are duly justified by the gatekeeper.
So Apple just needs to explain security & encryption to Brussels attorneys to keep iMessage in a silo, for example. (Obviously making iMessage interoperable with e.g. Discord or ICQ will compromise the integrity of the software features of iMessage.) I don't think this is going to increase the pace of product improvements.