Intervals of course are the recoveries, the repetitions were the hard word. So, he referred to this as anaerobic training. Not "speed work" and not "interval training."
He learned from others and self experimentation. Jack Dolan was a great influence.
I see someone mentioned to "improve Snell's speed," later in this thread. No, Snell had the slowest 200m personal best on the start line of the finals in Rome and Tokyo Olympics. But with 200m to go (in the 800), he dropped everyone. So, the question was how, if he was the slowest over 200m?
The answer is that the 800m is not about muscular speed. It is about having the aerobic conditioning to take advantage of whatever speed one may have. Look at Keely Hodgkinson, although not a volume based athlete, her 400m is much slower than Mu and Moraa's yet she beats them on occassion and like Snell has Olympic gold.
Volume was important for Arthur's athletes, but when asked which phase of training is the most important, he said, "everything." Interestingly one of the more overlooked phases of his training is the hill phase. Ironically, the hill phase's purpose is to develop power in the foot, ankle and lower leg and activate the lifting muscles in the upper leg, this, to prepare the athlete for anaerobic training on the track. To develop the power to top up the limited anaerobic capacity.
What do we have today instead of development of the foot, ankle area, lower leg? Super shoes that do all the work.
But to answer your question, I fully support HRE's well-informed answer.....