It's literally not. And your analogy is bad, because it describes inelastic demand, and not the platform effect.
To me that's semantics.
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.
How is the Play Store not a monopoly? It has been found such by multiple judges in multiple jurisdictions for a reason.