They actively prevent developers from building their own app stores and competing with apple's own apps.

Bell telephone had a similar monopoly. They owned the phone lines and wouldn't allow any non-approved equipment connected.

Your view really depends on if you see the phone primarily as a platform for apps or as an "apple phone" with some apps as neat little add-ons.

Antitrust comes about when a company has a high enough market share AND uses this market share to bludgeon any competitions. Apple created a market, they own the method of distribution, and they actively use their position to prevent competition with their own products and stop alternative markets from forming. Much like Bell