I think they are dead on in the article. It makes complete sense, which is why I always laugh when people tell me they go into neutral to save fuel.
I drive a MT car and I always gear down as I decelerate. It saves on your brakes as well as saving fuel. Many argue that it shortens clutch life, but on my current car I am at or above the average life span many others report.
Unfortunately I can't compare my numbers accurately on here, but on a forum devoted to car & engine type I am on the high end of MPG when comparing numbers.
And honestly, if shifting to neutral saved that much fuel and caused no safety risk, don't you think auto manufacturers would make the car do so automatically?
I do know by using the guage on my car (which I've found to be fairly accurate comparing my calculated numbers from here) I use much less fuel coasting down a hill in gear, than when my car is idling.
There's no need to downshift unless you'll be coasting down a steep hill and you need to keep your speed down (as you do use gas to rev-match, but you use zero gas coasting in any gear as long as you're above 1500rpm or so).
But there's no doubt that coasting in gear takes zero gas and coasting in neutral takes a little bit of gas. If you're going to be coasting down a slight hill for a long ways and leaving it in 5th gear will cause you to slow down, that is almost the only situation where you'd want to be in neutral, but those hills are few and far between.
Otherwise, the vast majority of drivers are usually coasting towards a stop light, traffic, or a turn, and need to reduce speed anyways, so leaving it in gear both slows you down and uses no fuel. But the amount of fuel used while idling (especially in new cars) is so small that it won't make a huge difference either way.
Living in Wales, i have experimented with this. Obviously being in top gear and NOT accelerating uses no fuel as the engine management cuts fuel on the over ride. But it uses fuel to idle (e.g. if you're in neutral) I wouldn't recommend putting a car in Neutral if it has an automatic gearbox though (urgh!)
However there is a hill near me that you can travel 6 miles in neutral. You cannot do this in top gear cruising as the revs and resistance in the gearbox slow you down! So you can either travel further but maybe use more fuel, or use less fuel but not go as far...
Your MPG when coasting in neutral is still very good (30-80mpg depending on speed) but the fact that you could be getting infinity mpg (using no fuel at all while still traveling) means you are wasting a little fuel.
Modern auto transmissions are starting to get better MPG simply because they have more gears and at least one ultra-long cruising gear. Since they shift quickly and automatically, you can maintain speed at very low rpm with a small motor at high speed, and the driver won't care because it will shift to a lower gear as soon as they want more power. Drivers with manual transmissions don't tolerate as long of 5th/6th gears simply because they don't like downshifting whenever they need to gain speed. Note that the autos still all have lower city mpg ratings than their manual-trans counterparts. The OEMs figured this out a long time ago, but that was when extra gears in transmissions were more expensive than CAFE penalties.
Neutral and engine off saves fuel. Not saying its a good idea but if you are that concerned about the fractions of gallons and dont mind suddenly not having power steering (or brakes) then give it a go. Although it is going to take so much muscle at that point you might as well be riding a bike. Or pushing your car.
Like everyone has agreed staying in gear with a "neutral" throttle position, not engine braking but close to it, provides maximum sensible fuel conservation.