Issue with `Opening a Full Screen Modal`?

The website documentation has a section on "Opening a Full Screen Modal" (here).

The method described is to have two child Stack Navigators with a shared parent. The children are created with mode: "modal" and mode: "card", respectively.

Given this setup, let's assume a user opens an app to a card Screen. As soon he navigates to a modal screen, wouldn't the modal StackNavigator contain an unwanted screen at the root of its stack -- the screen that StackNavigator created by default as the initial route? Of course it's possible to provide an initialRouteName, but in this scenario, it doesn't make sense to do so. The modal navigator's screens are not meant to be shown in succession, and hence, this navigator has no conceptual initial route.

Is there a way to address this that I am missing? It seems like the documented approach would create problems with back behavior given the existence of the initial screen at the root of the modal StackNavigator.