My experience is the opposite- all cruise controls work to maintain SPEED, not engine load. As such it doesn't care about eco or not- it does what it needs to do to maintain XX miles per hour. By coincidence it may be maintaining ECO better than your foot can.

Having said that, initially CC probably will get you better mpg than your foot, until you train your foot to achieve a light touch. Once you get a feel for maintaining LOAD as opposed to just SPEED, your mpg shoots through the roof.

Case in point: 33 mile rolling hill commute, >80% highway. 65mph nominal.
With CC: 38.x mpg.
Untrained foot (when I 1st bought it): 36mpg.
Now, with trained foot: 41mpg on a bad day. Yeah, my speed drops to 60 on hills, but the engine load is more constant.