> That's incredibly tautological.
It's not, because markets are defined by economics and not brands. People keep asserting over and over again that a brand is not a market, but nobody is claiming that, they just happen to coincide in this case.
What defines a market is whether there are substitutes. If you have an iPhone and need an app, and you could install an Android app on your iPhone, then iPhone apps and Android apps would be the same market. But since you can't, they're not.
Notice how this doesn't apply to random Walmart products. There may be a Walmart brand of motor oil that only Walmart sells, but you can go to Amazon and buy a different brand of motor oil and still use it in the same car. The relevant market is "motor oil of the spec that works with your car" rather than "Walmart motor oil" and Walmart doesn't have a monopoly on that.
But for the market "apps of the spec that works with your iPhone" you can only buy through Apple, so they do have a monopoly.