not as antitrust laws are written, no. not a sherman section ii monopolization of trade or commerce, not a section i restraint of trade, not a clayton tying. as with most antitrust cases, it comes down to market definition. the reasonable market is phone messaging or some such; the only way to win would be basically defining the market as imessage itself which no court would suffer since whatsapp and similar are free with equal or greater features.

you can argue antitrust statute should be expanded or changed, but there's no reasonable case as things stand.

It would be if they had a dominant market position. Just controlling the endpoints doesn't mean that much by itself.