That sounds like it could be correct but I don't know where the information is. I did a quick check of the owner's manual and didn't see it. I don't recall that it slows going uphill but I'll try to remember to keep a closer watch on it and see. I think you are definitely right about getting better mileage if you let it slow when going up hill. I usually don't do that on highways where there is traffic behind me though.
So far I have been using ECO on the highway only and when I am able to lock in CC. I will try to work it in a lot more often and see what happens.
Works wonders down to about -10°C/14°F... after that, you'll need extra help than the ECO mode and exhaust heat recirculation system.