I think the argument is the bundling of iMessage in with SMS. Yes, there's lots of third-party chat services too, and yes, no other tech company is clamoring to allow unauthorized third party access either.
But iMessage gets to intercept and pretend to be SMS (or more accurately, iMessage exists to block the RCS your phone line always already had) and it's pre-installed, uninstallable, and with special access exclusive to apple, which is where I think the argument could land.
A case could be argued of iMessage + iPhone is really similar to the late 90s era Microsoft Windows + Internet Explorer, in that they both break Sherman Antitrust Act: Section 2, in very similar ways, for very similar reasons.
(to be clear, I don't think they'd win, but I could see a strong argument there)