For the App Store, which is what matters to developers, revenue was around $25 billion in the first half of 2019, vs $14 billion for Google Play Store.
It's not literally mono- but as the article notes, just having a significant share of the market (I would say 30% would be enough) allows companies to dictate prices, and iOS has more like 2/3rds of the revenue.
That's not even getting into all the restrictions that Apple puts on third-party developers, including categories of apps that you can't even make if you wanted to -- unless you're willing to sit out half the market, and assuming that Google doesn't do the same thing. As a software guy, I consider that a bigger deal than the % cut (which is not entirely unreasonable given how much of that goes to credit card fees for low-dollar-amount transactions)