Earlier quoted context omitted.
No, US antitrust law does not allow a single company's product to be considered a relevant market unless very specific criteria are met. Epic attempted to make this very argument in court and failed. "iOS app distribution" is not a valid relevant market for antitrust purposes.
You haven't been following Google's case closely enough.
Edit: In case you're wondering why "Android app distribution" can be considered a relevant market whereas "iOS app distribution" was rejected even though they both sound like the same thing, the key difference is "iOS app distribution" is a single-brand aftermarket, and single-brand aftermarkets have stricter requirements in order to be accepted by the courts.