"Interval training" is the new "Focusing on Core Strength".
The amount of calories you burn running a mile is more or less invariant with respect to speed, unless you are running at literally maximum effort (like 100 m dash-style), in which case you'll fatigue before you covered nearly as many miles as you would have running at a slower pace anyway.
I'm not saying interval training is a waste of time, it has its place in any well-rounded fitness program, but to argue that it's the fastest way to burn calories as a general statement is silly in my opinion.