I know you have done measurements so I do not doubt your findings but my question is how can that possibly be?
HLG only diverts from Rec709 over a certain luminance level, below it is exactly the same as Rec709.
So the question is how are the blacks and shadows encoded in sufficient detail given the fact that they are so far removed from the PQ curve?
How do the blacks under HLG behave? Do they gracefully disappear into the noise or are they compressed and clipped?
This Wolfcrow video https://www.youtube.com/watch?v=JdKwfEv145A I found useful in that I get that the HDR expansion into the shadows for the average viewer is not visible. So the highlights are where the average viewer will see the change. In this regard I expect that HLG has this advantage. That is my understanding at the moment. I like shooting at 60P so for me I will have to get an Atomos Inferno to shoot HLG with my GH5. On my list !!!