Not quite, because "sedans" is not the relevant market. It isn't that you can't buy a Kia the next time you go to buy a car, it's that once you own a Civic you have to put Civic replacement parts on it. So then you get the relevant question, does Honda have a monopoly on any Civic replacements parts? Some parts (e.g. brake pads) may be produced by third parties in competition with Honda, so for those parts the answer is no. For other Civic parts not made by anyone else, the answer is yes.

So to get back to Apple: They don't have a monopoly on smartphones, they have a monopoly on iOS app stores.