True, but I think the sentiment is correct at least in selling food. It's hard to get rich selling spinach, but it's easy to get rich selling Sugar-Frosted Excitement Pops - now made with whole grain-like substances!
Fad books sell to a lot of people over and over. Of course you can get rich helping people to just eat healthy, but it's harder and you likely don't get the repeat buyers. But write a book saying x is evil or fat is not your fault because y is evil and you'll get tons of buyers.