Seems to me that even with the HDMI port disconnected, the Pi thinks that a composite display is connected.
Sorry, but I have no idea how you can convince the OS that you're using a DSI display and whether that will work without X11/Wayland on the buster. I noticed in the RPI docs that there's a KMS type overlay for the display which would make it work without X11/Wayland (as required by RetroPie), but I'm not sure whether that would work without installing the lateste RaspiOS (which is based on Debian 12 bookworm).