Isn't that the wrong way? In neutral, your engine uses fuel to keep idle speed, but decelerating in gear uses no fuel at all. It's definitely this way on Audis, as it in the '800miles on a tank' episode of Top Gear mentioned earlier.
I don't know what the episode mentioned, but have seen some stuff wrong before.
Let's consider a normally aspirated car for a minute. A lot of times, the increased momentary fuel consumption is associated with Lower (deeper) vacuum. If you look at the vacuum gauge that some normally aspirated cars have, when you suddainly floor it, the vacuum will really go deep. That's how theose "economy" gauges some cars have are hooked up too.
My point is that one of the deepest vaccums is when you use the engine to decelerate.
Even with a turbo car, the same applies. It will not be as bad of a consumption as in wot but right up there.
As far as consumption in idle, that's actually the minimum consumption. With carbureted cars there used to be a separate jet, just for idle. The main emulsion tubes used for normal driving are not even in the same category size wise.