So "monopoly" as a single entity controlling a single market is a simplistic view of the issue at hand. Anti-trust is far broader than that, where any anticompetitive action can be subject to anti-trust lawsuits/regulatory action.

So the legal argument would be that, because Apple allows for a single messaging app, and interacting with that app requires an iphone, they're effectively preventing messaging app competition.

Apple has a very, very talented legal team though, so, for this to even see argument in court someone's going to have to realllly have to want it, and be able to fund it.