The problem isn't the messaging service, the problem is the artificial hardware requirement in order to use it. Second would be the inability to make another app the primary/default once you have said hardware.
This suit seems to follow the interpretation of: "it is bad if consumers are being harmed in some way". Having a monopoly position via market share is not a necessary condition for that to happen.
People excluding people is the problem. Not the product.
The Americans all say that to get in, you need an iPhone. So definitely a smart monopolistic strategy from Apple.