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
That is one of the extensions, in theory others could be built. RemoteFX is an extension as is the (more normal today) Desktop Composition Extension.
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.