I have no idea - I am not a mechanic or mechanical engineer; however, this is what I was taught to do in the Army (and three of the four vehicles were driven by ex Military), so I always do it after driving on a hot day.
Having said that though, this photo was taken at 6:00 pm that evening and it was still 37 degrees Celsius and there was absolutely no breeze and in those conditions I don't think that I would have the motor running with the heater on high and no air flow going through the radiator, seems to be a recipe for a disaster. It might work in cooler climates but in our hot and dry conditions I am not so sure and TBH, I won't be testing it out.
Karl
I'm not trying to change your mind, when I was given this info, I said pretty much the same things you are saying. The reply was that at idle with the hood closed, the fan draws plenty enough air to cool a hot engine down. Having the heater on draws even more heat away from the engine. I took an RV driving class taught by a former long haul trucker. That instructor, with over 2M miles, said that 10 minutes at idle, even with the heater off, was enough to bring a hot engine down to low enough temps to turn it off. But if the hood was open, it would not. Today's systems are sealed better than yesterday's, and with the hood open, it cuts air flow through the radiator, which is the main cooler for the engine.