In fact the legal system punishes you for having an open platform. The very reason the author can leave the play store is the same reason why the play store is a "monopoly". So sure, maybe the play store is a monopoly with some rigid definitions that lack nuance, but lets not throw away logic here.
Something like the DMA and gatekeepers exist in part exactly because of the status quo and entities like Apple. I personally see the gatekeeper approach as a solid base, but know that it can be quite unpopular within the industry.
If I had an app, I'd have to pay Apple whatever they want to charge in order to get access to one part of all mobile phone users and Google whatever they want to charge in order to get access to the other.
Maybe calling that a monopoly wouldn't be good enough in an economics paper, but I don't see much of a practical difference. I get to accept the price or not do business.
And to set off some more bots: We desperately need regulation in this space.