You don't know what a walled garden is. A walled garden, or closed platform, is a platform that prevents you from going outside the ecosystem, or platform, in order to use it's features.
To continue the supermarket comparison, it would be like a system where you buy pots and pans and a stove and what not from Whole Foods, but you can only use food you purchased from Whole Foods, and anyone wanting to sell you food would have to sell it through Whole Foods.
Open to closed is on a continuum [1]. What we include or don't include in our analysis is a matter of preference. As such, there is no one true definition of a walled garden.
For example, Apple doesn't dictate which electricity provider I use to power its devices. Does that mean it's an open platform? Of course not.
The comparison of the App Store to a grocery store is apt. They both leverage their access to demand to control (and extract profits from) the supply side.
Whole Foods has store brands. You can't buy Whole Foods' store brand items at any other store. That aspect of exclusivity is harmful to consumers, since if they want to get the store brand items from different stores they have to pay money (fuel/bus/etc) to go to each store.
Yes? Of course? That’s the whole point of a supermarket. To curate and distribute in exchange for a cut.