There is a reference to running up to 0.75 of MAXIMUM heart rate for Running Economy runs. I think that the refernce should be to Working Heart Rate (MHR-RHR) as the scope for difference is substantial with a difference of 30 or more beats a minute RHR (beginner runner to elite athlete.
Is this what is meant in the article ?
By the way, tries the VO2 pace - felt bang on cheers !!