Rubbish.
A modern engine management system uses no fuel on overrun. That's why the mpg readout goes sky high as it's literally running on fresh air.
Idling in neutral uses fuel to keep the engine running whereas leaving it in gear and off the throttle makes the car 'drive/turn' the engine.
whilst that's true, you're using the kinetic energy of the car to drive the engine at 2,500rpm (say) instead of using the energy in your fuel to drive it at 700rpm. leaving it in gear uses more energy. If you're stopping at the bottom of the hill anyway, then that's fine, however if you're just driving up the other side of the hill, you may find dropping to neutral is best, purely from an economy perspective.
there was a report a few years back that proved for a given car (polo diesel rings a bell) the most economical way of getting up to speed was to floor the throttle in each gear, up to 2,000 rpm, then shift up. That way you were in a lower gear for less time, which increased economy.
The whole revs thing is engine dependant. on most engine, the lower the revs, the more economical it is. On some petrol engines, with high cams, you can end up pumping unburned fuel into the exhaust this way, which isn't good, but this isn't possible to do on a diesel engine.
lower speed helps hugely, that (whilst you're still in top gear) and staying off the brakes are probably the best things you can do
and yes, too far to walk in a reasonable time and no bike was available.
