The real question is if a given "Apple product" is a separate market. Is it fungible with a competitor?
If you ask this question about "Clorox bleach" the answer is obvious. Any other brand of bleach is a perfect substitute and the only reason Clorox can demand a premium is if customers value the brand as an indicia of quality, i.e. they're paying to get bleach they trust not to be watered down or contaminated in a bottle that doesn't leak, not because there is no other source to get bleach they can use with their washing machine.
For the iOS app store that is not the case. If you have an iOS device, you have to use the iOS app store. It would be as if you had a Kroger-brand washing machine that could only use bleach from Kroger, e.g. because they sell it in cartridges that authenticate with a microchip. At that point Kroger would have a monopoly on bleach for those washing machines, and that product would be its own market because there is no competing substitute.
For this not to be the case you would have to be able to install apps on iOS devices via Google Play or some other competitor with non-trivial market share. Because the relevant concern is whether there is a viable substitute that the same people can use for the same purpose.
And yes, this means that if you cordon off something that might otherwise have been part of a larger market by using DRM, you now have a monopoly on the thing you erected a fence around and should be subject to antitrust enforcement.