No, I mean the "Apple has less of leg to stand on than the carmakers would with that idea" part. It's Apple's store, they do what they want with it, and they charge 10% because people are willing to pay it. Carmakers don't own the refueling stations, except for Tesla in some cases. There can be intervention for anticompetitive practices like discriminating against competitors, but no central authority decides things like transaction fee percentages.
It's Apple's store that gives an opportunity to stream music. And it's the carmaker's product that makes gas useful.
What Apple is doing is absolutely anticompetitive. You can't load an app into your customer's phone except Apple's way. It would be the same as the carmaker only allowing gas to be filled their way. And there are plenty of artificial technical ways they can accomplish that, just as Apple's way for apps is an artificial limitation enforced by encryption tech.