The above Overdrive5 API seems to be the appropriate one for querying the GPU activity. I don't have an Overdrive5 supported adapter, so I can't verify it myself. However, as I ran the Overdrive sample on an Overdrive6 supported adapter, it worked as expected. In fact, I modified the code to run it inside a loop. Then the printed activity percent was correctly changing based on the GPU load.
I would suggest you to install the latest driver and then run the sample. If you still don't get the correct value, please report the issue here: Issues · GPUOpen-LibrariesAndSDKs/display-library · GitHub
however in the sample it says that this is the current engine clock.
Looks like a typo because corresponding Overdrive6 code prints it correctly as "Current GPU activity level".