It's an old concept and it wasn't even true then.
Wiki:
Salazar's competitive decline is often attributed to the stress on his body from that memorable Duel in the Sun detailed in the eponymous book by John Brant.[10]
Salazar recounts falling into a "more-is-better" mindset which led him
to reason that if 120 miles per week yielded a certain level of success,
then 180 miles (290 km) or even 200 miles (320 km) would bring even
better results. This intense and grueling regimen of such extremely long
distances led to a breakdown of his immune system, and he found himself
frequently sick, injured, and otherwise unable to continue training.
After failing to make the 1988 Olympic Marathon Team Salazar opened a successful restaurant in Eugene, Oregon.
Although only able to stagger through four or five miles per run, he
remained obsessed with training. Brant wrote that "He couldn't run, yet
he couldn't stop running." Salazar unsuccessfully visited the Stanford
Sleep Clinic and a cardiologist, had surgery, and trained in Kenya. In
1994 he said that "For most of the last 10 years, I hated running. I
hated it with a passion. I used to wish for a cataclysmic injury in
which I would lose one of my legs. I know that sounds terrible, but if I
had lost a leg, then I wouldn't have to torture myself anymore."[10]