You're incorrect: there might be some decoration provided by the app, but there's a layer outside of that which is configured by the window manager. That's the title bar, and the little icons by which you maximize/ minimize/ "Iconify"... do people still do that?..
That's something that's different between X11 and Wayland - in Wayland, windows are responsible for their own decoration (I presume Xwayland handles decoration for X11 windows running through Wayland). This makes it quite a bit harder to hide all decorations on Wayland - various UI toolkits have ways that the end-user can tell them to hide bits, but they all operate differently and don't cover everything.
Plus thumbs at the corners, for some displays.