I'm not surprised if people find it more economical to slow down on hills - that gives a lower average speed and a lower speed gives greater economy. Setting the cruise control to that lower average speed would be even more economical.
I suspect the hill advice comes from someone looking at the instantaneous fuel economy readout - it certainly sounds like it. It's very hard for folk to understand that the best mpg reading now may not be the best overall - after all the best possible mpg reading comes from the very worst fuel economy behaviour, using the brakes.
There are two separate factors at work: how much energy the car needs to move and how efficiently the engine can turn fuel into energy. Contrary to what most people think, engine efficency is best at full throttle and peak torque, so pressing the accelerator pedal less is not a guarantee of better fuel economy.
That's why the real fuel economy experts, doing things like the Shell Eco-marathon at several thousand (yes, thousand) mpg use a burn and coast technique where they accelerate hard up to 20mph, switch off their engine and then coast down to 10mph before starting the cycle again. Doing this in normal cars gives fuel economy in the hundreds of mpg - but it's a crap way of getting anywhere.