This is an odd saga in the history of Lydiard training. I think it was in the 70s and Arthur was talking to an Australian physiologist who claimed that 100 mile weeks were not enough to maximize aerobic development and that you needed to be around 200 for that to happen. Usually Arthur's response to people who told him things like that was to ask them how many athletes they'd coaced to Olympic medals or world records and then ignore them. But for some reason, this physiologist seems to have struck a nerve so Arthur responded in the way you mention and went on to include this episode in some of his books.
If you check with the guys who trained with him you'll find there is at best a only very slight grain of truth to this story.
The story annoys Snell who is adamant that he never ran anywhere close to 200 mile weeks. One hundred was just about the top for him he told me and said that "we counted every step." Barry Magee told me he topped out around 130, counting "supplemental jogs" and Barry was one of the higher mileage guys of the group.
Arthur had one athlete, Jeff Julian, who did get to and beyond 200 mile weeks and Lydiard himself had gotten that high when he was experimenting on himself. Later, Richard Tayler got onto the high side of 150 but aside from those guys, there is no corroboration from anyone who ran with Arthur that anyone other than Lydiard and Julian got to 200 mile weeks.