The swipe in is a system input for back action. It works on every app and between apps, and it works anywhere on the right side of the screen, instead of having to reach wherever your app back button is, if it’s not collapsed and has to be scrolled in to view as well.

If you look at the screenshot, it doesn’t even have a back button. Only a close button. Whether they navigated or not is not clear, but likely won’t change the header controls in this view.