Just to add my interpretation of what Magness would say, which is: intense interval training tends to show significant gains over moderate work in shorter (8 weeks) study periods, but those gains level off quickly in the long term. A more blended training approach that incorporates all types of training intensities (hard, moderate, easy) will outperform just doing hard interval training, over the long term. And this advice applies to all types of athletes, not just people training at a high level.
Yes. And he emphasizes spending most of your time doing easy volume. Because especially as a regular Joe you won't recover well from doing high intensity intervals three or four times a week, especially when you haven't built a strong aerobic base through lots of easy volume.
And Magness isn't saying anything revolutionary here. Every other coach says very similar things. But Magness has the credentials to make him more authoritative than a random coach out there.