Apple Intelligence was announced at WWDC two years ago, they had plenty of time to work on interoperability. Besides that:

access to all of your messages, photos, what's on your screen, browsing history, etc. Apple says hey so we're going to need some time to figure out if we can do that in a way that won't completely fuck over our users.

The point is that if Apple's model gets all that access, they should give others access to those APIs as well, otherwise they are giving themselves benefits over the competition. A company can do that, but not once they are considered a gatekeeper in the EU. It's up to the user to choose an LLM provider that has good privacy rules (or stick with Apple if there is no other provider). That's fair competition, a user can weigh pricing, privacy, etc. and make their own choice. Now they are stuck with Apple and have to get an iCloud+ subscription to fully use the AI features. The 18 month delay is not to figure this out, it's to entrench themselves as much as possible first.

Following your line of reasoning, if Apple had this behavior in 2010-2015, instant messaging applications outside iMessage wouldn't have the option to ask access to your contacts (privacy), no possibility to share a location in a chat (privacy), no means to show notifications (probably privacy too), etc.

It's surprising how much people are willing to do the bidding of tech oligarchs. Remember, this is the company that has spent years doing malicious compliance around the DMA and DSA, why should they be trusted this time?