No, I am very clearly not.

Everything a company does to protect its position isn't "anticompetitive behavior." That term has a specific meaning in US law, and applies generally only to situations where market manipulation is being attempted (or committed) by a monopolist or a consortium of firms large enough to act as one.

See

https://www.ftc.gov/enforcement/anticompetitive-practices

Neither of those situations applies to iOS and iMessage.