He wasn't overweight, or living on pizza, candy and soda? I ask because people with that sort of attitude, in my experience, tend to take shortcuts on diet as well.
Lifted weights, ran nearly every day, ate a Mediterranean diet, low body fat, trim waist, healthy body weight, and a decent amount of muscle. He also did not eat much during the workday, so was effectively unintentionally doing intermittent fasting as well.
Ironically, one of his reasons for sleeping less was to have time to fit exercise into his hectic schedule.
His diabetes manifested as unexplained weight loss and muscle wasting.
So, no, he was not your "typical" diabetes patient.
Everyone else is probably safe trying to get as lean as possible so long as they're not malnourished.
15% and you see hints of sixpack (clear outline) for most people who train.