I think they’re getting away with it because of the phrasing.

Messages sent through the iMessage service are encrypted, but not all messages you sent through the Messages (note the lack of i) app are iMessage messages. It’s the exact type of fucking sneaky bullshit that should be regulated so hard it stops existing, but I guess our regulators don’t think it’s a big deal right now.

@edinbruh@feddit.it mentions in another comment that

A recent … EU law mandates that if your platform is big enough … to gatekeep users from using other platforms, then it must interoperate with competing services.

And I think this is how Apple would “sneak” through this as well. The Messages app doesn’t lock you into a communications protocol. If the recipient has iMessages, it sends via that, if not, it sends via SMS/MMS. No idea if that argument would hold, I hope it wouldn’t. I would honestly prefer it if there was just a single open messaging standard that anyone could hook into, because closed proprietary tech is fucking bullshit on every single level.