I have personal experience outrunning a bad diet as a division 1 swimmer. I was on a 7000 calorie a day diet, which was actually difficult to pull off, and I specifically had to supplement my diet with things like snickers bars and peanut butter cups just to stop losing weight. In fact, my dietary habits formed during this period of my life, where I was consistently below 10% body fat, continue to cause me trouble today in my less active state. Only by eating dramatically healthier have I been able to approach 20% body fat today.

Even beyond myself, I think you’re romanticizing how healthy the diets of extreme athletes are. I’ve been coached by and trained alongside Olympic athletes and most of them (not all of them) don’t give a single shit about things like healthy fats or micronutrients. Protein definitely, but everything else is noise. When burning that many calories, you are getting more than enough micronutrients, and it doesn’t really matter if the energy you end up burning is from fats or carbs, because it’s in and out the same day and never has a chance to be stored in the first place.

Body builders aren’t judged on athletic performance but aesthetics. It would make sense they care a lot more about diet, but it should be noted that they aren’t athletes and their entire regime is about building muscle, not using energy. It’s a completely different type of optimization.