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
The Desktop Composition extension was never used in practice, check the date on the spec - the last update was in 2017. I don't know the whole story behind the scenes but the feature got pulled early.
