I believe the author you are referring to is Bertram Miller. In an article written by him he says that C-41 developers he scratched mixed never did equal Kodak Flexicolor developer in terms of sensitometric results. But he found that if he used part A of the Flexicolor developer and mixed it with scratch versions of parts B and C, the sensitometric results equaled Flexicolor developer. This led him to believe that part A had some proprietary or "secret" ingredient(s) not included in scratch formulas since they are not generally known.
But as PE has pointed out, grain and sharpness are also factors in which scratch formulas likely do not match Flexicolor developer due again to missing ingredients. I use both Flexicolor and home-brew scratch chemistry and have found my particular formula to have sensitometric results that are close enough to Flexicolor to satisfy me, and while I don't measure grain and sharpness, visually I am satisfied with the results I get; others might have more stringent requirements. I mostly use Flexicolor, but I want to be prepared for the day it becomes difficult or expensive to get or completely unavailable.
rpc