I would say that in practice 99% of people can't outrun a bad diet, but not because of any sort of physiological reason. You simply need to train so much that most people won't ever approach the level of running/cycling/lifting they would need to do so.

If you're training like an elite athlete (for me and my at the time roommate that was running 85, or in his case, 100+ miles a week with a few lift sessions) you can, and will, eat just about whatever you damn please and not gain weight. Most people can't fit that much training into their lives without making it their life's primary focus at the expense of everything else, and couldn't sustain that level of training if they did, so it becomes a practical impossibility.

I do miss that aspect of running so much mileage, though I appreciate the freedom that stepping back from competition has afforded me in other areas. To maintain weight now, I eat 1-2 meals a day, but back then? I ate whatever got put in front of me, sometimes 4 meals a day.