I've never heard that myth.
Calories burned is kind a guestimated science, as everyone's body reacts differently.
One thing to keep in mind as you lose weight, is that because of the weight loss the same activities don't burn the same number of calories. This is one of the major contributing factors to the "plateau" effect people have. If you hit a wall with your weightloss, work out harder, or longer, to burn more calories.
The difference for a 250 pound man between, walking at 3.5 mph for an hour, and 4mph for an hour, is just 150 calories.
But the difference for the same man walking at 4mph for 30 min vs 4mph for 1 hour is 200 calories.
Lately I've been focusing on calories burned for the day vs time/distance, etc.
I do an hour on the treadmill with varying speeds, but never below 3mph, and that's usually just for a short period of time to get my shin pain to subside enough to step back up to 3.5-4mph.
Today, according to the treadmill readout, I burned 670 calories in my 1 hour cardio session with a 5 min cooldown.
Considering my dinner of a footlong oven roasted chicken on wheat sub from subway is about 700 calories, it's like a free meal