Mentzer didnt eat "whatever". He liked food, but due to a very slow metabolism, had to diet very hard to get ripped. He attributed it to bulking up too much in his teens- at 17 hit 250 lbs, able to squat 500 and bench 450. Even when he was just training people, He was eating fairly basically, when I was around him, one lunch was turkey sub- no cheese, not chips. Dinner was Chinese delivery or such. Fairly low fat, medium protein. I also heard he could devour a 7 lb lobster. Missed that one, dammit!
He would typically eat his carbs early, train, and the rest of the day was chicken,, turkey, fruit.