The RDP GDI messages are from RDP6, graphical messages have changed many times over in the versions of RDP that followed. Would you be surprised to learn that WSLg uses a FreeRDP server hooked to a Wayland compositor running in the Linux guest, and that the RDP client in the Windows host is MSRDC in a special RemoteApp mode? In theory someone could build a similar RDP server using IronRDP