This is correct but I wonder how people say higher consumption.
The problem is staying at D causing the car to go downhill too fast but shifting to 3 makes the car go slower than necessary. 
So using lower gear then need to press gas pedal causing high rpm maybe the cause but not much power needed by engine still. I usually stay on D and apply brake once in a while though you still need to judge whther 3 or D gear is the best depending on slope.

But all the autos I have driven dun auto downshift in idle downhill unless told to (i.e. D2,D3) - unkers own ride is manual sendiri can apply on demand la... The ECU know its going downhill meh? Or measuring speed vs no throttle? Mebbe is new feature kot. Unke rprob diving 20yrs old auto haha

Are you sure change to neutral? I don't think so. Even pro drivers said DO NOT cruise down hill in neutral.  This is dangerous practice. There is reason to it. 

First of all, understand what is engine braking.
Once you understand it, see this youtube link from Engineering Explained.

https://www.youtube.com/watch?v=_bZlb62VVlw

There is 1 exception unless your car is older type which use carburetor, then it will consume higher fuel as it is not control by ECU.
But modern rides, usually will cut off the fuel when doing engine braking.

Per video from Engineering Explained, the engine cuts the fuel injection when the car moves down a hill; at various times, the engine will use no fuel at all to power the engine, since the wheels themselves—connected to a transmission—make the engine spin.

How do you know if the car is not put in N?    Below 1k rpm is engine on idle which normally when car in P or N position.
Safety rating is just rating. It is still not safe if you drive dangerously. Do you know that coasting (or going downhill in free gear N) is even illegal is some states in the US?