I think what Dino is saying is that obviously you aren’t going to gain a lot of muscle just by eating like a bodybuilder and not training. The training has to be there or you’ll just get fat. However, a person can lose fat without training and through diet alone (not a good idea, but it works.)
Diet was chosen as the number one factor by many people in the article because it’s often the missing ingredient. Not getting big, but you’re training hard? Diet. Training hard, doing a lot of cardio, but not losing fat? Diet. Progress stalled? More often than not: diet (although there are other factors here like poor training, genetic ceiling etc.)
Basically, when I’m helping a person reach their physique goals, we start with diet. Many worry about (and get anal about) the specific type of weight training they do when trying to get into shape, when in reality, that’s only a secondary concern over diet. It’s important, but not the #1 priority, or rather, the specifics of training aren’t the number one priority.
It’s like triage. When a person comes in bleeding to death from a gunshot, you don’t treat him for possible infection first. No, first you stop the gushing blood coming out of his chest (diet), then you take care of the other stuff (training.) I’m not saying that you don’t have to train obviously, but the tiny details of how you train with weights aren’t as important as what you’re eating in terms of reaching those goals.
In short, you can’t “out-train” a poor diet. Get on a treadmill, type in all your info, and let it estimate how many calories you burn in an hour. Now see how many calories are in that candy bar and soft drink “snack” you had that day (or in the beer you had the night before.) Scary.