I don't get it. The screenshot on reddit appears to show that tapping on the card changes the billing info, and under that there's a separate button to change the card. So far as I can tell that's the same on iOS 18? The only difference is that tapping on the card doesn't do anything. What's the "muscle memory"?

iOS 18 is what introduced this. In iOS 17 you could tap the button with the card to change the payment method.