They absolutely are. Apple makes money from the iOS ecosystem in two ways: through device sales, and through App Store purchases.
I'd agree that iOS is an example of a restrictive platform, in that ~30% of all revenue goes to a single gate-keeping party.
In any case, I messed up the quote a little, another commenter found the source and real comment: 'Gates said something along the lines of, “That’s a crock of shit. This isn’t a platform. A platform is when the economic value of everybody that uses it, exceeds the value of the company that creates it. Then it’s a platform.”' [1]