Only his answer is wrong, Vabsie. The examples he gives are not relevant, as they are not typical light aircraft. I am not sure why he mentions single-seat agricultural plane I have never heard of in 20 years involvement in a variety of sides of aviation to a question in private flying.
Typical light aircraft have a much better glide ratio than he suggests, which is important to people who are planning flights, especially if there are areas where an engine failure would lead to a difficult landing - over water, woodland, mountains or other rough terrain. I have a friend who managed to glide just clear of the rough moorland hillside he had an engine failure over. Poor glide ration would have meant he could not have made the glide off the hills, and had he thought like that he would have concentrated on the wrong thing, minimising impact in a poor location, rather than achieving a safe landing in a good one.
SN3 also concentrates on prop drag, which is a problem that can be overcome but is also not as dominant as he suggests. Stopping a propellor cancels out 95% of prop drag, and can be achieved in a light aircraft. However a decision has to be made to do that or concentrate on the glide. Shutting down an engine in a light twin all commercial pilots train in at some point is far more relevant as a demonstration of the prop drag of a light single (similar power per engine) than one of four large radials in a big aircraft. Prop drag is important, but really ain't as much as he suggests. If it was it would be really important to stop the propellor in the glide, but except in marginal conditions it is not.
SN3
Forgot to mention that with that large prop the airflow is much slower than the little prop on a normal light single, so the sum changes giving more thrust under power as well as more drag when windmilling. In fact airflow is something I reckon, on further consideration, I over-estimated in my original equation so the thrust limit is higher than I had guessed. However not by enough to actually let the aircraft fly in practice! In fact a scratch calculation which takes that into account, using just pure Newtonian mechanics of energy and momentum brings in some fundamental physical limitations to efficiency due to speed of airflow. Through a typical 78" diameter prop this gives an even lower thrust limit, around 2,200 N. So actual thrust must be well under 2000 N.
Worked with a larger prop the force gets higher of course (because energy relates to velocity squared, momentum only to velocity, so the same energy produces less change of momentum if the speed change is higher. Force equates to rate of change of momentum), and with a helicopter rotor diameter much higher still, which is how it can fly.