IMO everything has bias, why else write a book? This site is hugely biased, especially since people who cannot at least reasonably aspire to making a go of retirement will quickly drop out.
Unless we think this author is lying, or that he is directly and erroneously addressing us, I don't see any reason to discredit what he has to say just because it does not fit our own self images.
Again take exercise. A recent survey ( don't remember the citation) found that 25% or Americans do not take any exercise. They do not even a walk a dog, or walk to pick up some groceries, or walk to church on Sunday. Nada, nothing. Now this survey did not do age breakdowns, but I would guess that more than a proportionate share of these non-movers were retired if only because on average, retired people are older than average workers, and age brings more discomfort, again on average, not every individual. I was no longer living at home when my Dad or Mom retired, but we lived in a quality high walking urban neighborhood, and whenever I was around I never noticed anybody much walking other than my parents and much younger people- university students for the most part. Retired people who are very active as they age are always around, but almost never common. Walking golfers are high achievers, as are people who make walking a hobby. Like really long intercity walks
My same age friend that I have known since we were on high school teams together, then college roommates, and most recently regular lunch companions is now comatose in a hospital secondary to a surgical mishap. But really secondary to his sedentary life. He is retired, he has plenty money, he has a devoted wife, he has many friends and colleagues, he is an upper middle class PhD holder but he still is lying in a coma from which he may well not recover. He has gained at least 50 pounds since college, is diabetic, and from my pov has had extremely poor but by the ADA book care. He just had to spin the medical roulette wheel too often to do better than the averages, and the averages of by the book diabetes care are not attractive. The life of a typical diabetic passively following instructions often is a dispiriting downward spiral of losing appendages or functions, until something happens or he loses a function that he cannot live without.
He thought I was crazy to be always walking around town, adding ballast to my pack, rowing my Concept 2, or going to a gym. And he was a much better athlete than I ever could have hoped to be.
I'd say that there are likely several examples of failing at retirement on one or more dimensions among almost every older person's friends or neighbors or relatives, and of all the ways to fail it may be that not taking care of one's physical exercise and nutrition is one of the most important ways, or even the most important way to fail.
Over and over again it has been shown that some movement of any kind, at whatever frequency or intensity one desires, is more than half the battle. This is one more area where to get hung up on achieving the perfect is certainly the enemy of the good. Ironically, about exercise no one even knows what is better, let alone best, though all kinds of people assure us that they do in fact know.
Ha