> It doesn't guarantee that it will be hidden (the user may have accessibility or other options enabled which take precedence and prefer to display it)

In my experience as an iOS developer using this API, the only time the preference to hide the home indicator is respected is in the case you mentioned, when a full screen video is playing. Otherwise the home indicator always appears whenever the user stops interacting with the screen. So the GP is correct that there is no user control to make it hidden, and not really any developer control either.