Hi YorickStap - yes that's correct. The administrative setting will determine the configuration at all times, regardless of the configuration of the client. If you make the change at the GPO level against the computer object and perform a gpupdate /force, the session will immediately flip to "adaptive" from "h264".
If you have the Vmware Horizon Performance Tracker application installed on the VDI, you can see this under the "At a Glance" tab and "Encoder" as follows (the following was from my lab):-
Hope that helps.
Thanks
Andrew