XKCD's point stands as long as nobody has a monopoly on an important medium. Apple and Google effectively do have a duopoly on mobile app distribution, and mobile apps are an important medium for speech in 2025.
From the first part of your comment, it directly follows that neither Apple nor Google has a required monopoly on the important medium.
IMO, we need to stop thinking in this broken paradigm of "-opolies" (with its loaded requirement to define what constitutes a given "market") and look at the actual coercive power they wield through market stickiness. Apple and Google both wield much coercive power with regards to software running on mobile devices.
