Arthur was never above embellishing. I'm sure Joe was just repeating what Arthur told him. Arthur definitely had stretches of time when he did 200-250 mpw but not when he was 45 and it was long before he coached Snell, who was not happy that Arthur was claiming that he, Snell, ran more than 100 mpw at any time.
The original question just seems silly. For one thing Letsrun posters are a small segment of the running world and those running 100 or more mpw is a small segment of the overall LR posting population. And as others have pointed out, you don't just become a world class runner by training like one. Take 100 people and train them like Craig Mottram trained and if you're lucky one or two might get results like Mottram did.
Arthur never believed nor claimed that anyone who used his training was going to become another Snell or Baillie. He often wrote about how people who didn't have the abilities to be world class runners could still improve tremendously and get a lot of satisfaction from the sport. The only coach I can think of who turned 100% of the athletes he coached into world class runners was Peter Coe who only ever coached one athlete.