If you failed to restart the engine and intended to glide some distance it might make sense to stop windmilling the prop since it generates a lot of drag. But it seems to be theatrics here.
Especially if the engine is still making compression, a windmilling prop is extracting at least enough energy from the free stream to keep not just the prop but the engine turning... this is a lot!
I agree that energy lost to turning the engine must come from the airstream, don't underestimate how much energy can be lost just to creating turbulence in the airstream - after all, that's why you drop like a rock if you do a slip with full flaps. And a stalled windmilling prop will generate less turbulence than a stalled stopped prop.
The discussion at your link makes a good point though - the windmilling prop might not be stalled while a stopped prop certainly is stalled, and a stalled prop obviously creates less drag.
It is surprising that there's no good data on it. Someone with a scientific mindset in a remote location who's good at restarting a stopped engine in flight should run a couple of tests.
