But to make a good trade, you need to know the relative benefits of two parts of the advice. Giving up fats might be good and/or neutral; but if it exhausts your willpower to avoid sugar, and you indulge, then you are worse off.
There's also an angle about how this pushed a lot of people to heavily processed foods because leaving out everything on that bad list left a much narrower list of things you could make at home which were also tasty.
I can't imagine a scenario where that cereal/grain recommendation in GP quote isn't sponsored or populistic at the time though.
And while their method/reasoning wasn't flawed if they indeed had no research opposing their recommendation, that's kind of a straw man from the main issue of avoiding fat being legitimately bad.
You need energy, and you get some from protein which you desperately need, but you can't get all your energy from protein, and that leaves you fats or carbs. Of those it's so much worse to avoid fat than carbs.
https://www.washingtonpost.com/archive/lifestyle/1977/01/20/...
“Dr. Mark Hegsted, professor of nutrition at Harvard University and one of the three nutritionists who helped write the report, said: "There will undoubtedly be many people who will say we have not proven our point; we have not demonstrated that the dietary modifications we recommend will yield the dividends expected.
"The question to be asked therefore, is not why should we change our diet, but why not. "What are the risks associated with eating less meat, less fat, less saturated fat, less cholesterol, less sugar, less salt and more fruits, vegetables, unsaturated fat - and cereal products - especially whole-grain cereals? There are none that can be identified and important benefits can be expected."”
I find the history of that fascinating since the Nixon administration felt pressure to do something about the then-dramatic obesity crisis on a time-scale which was too short for the scientific process. They had some of it right (eat your vegetables!) but the big mistakes with fat and salt made the problem so much worse.