I do bouldering and I eat fast food multiple times a week, I can definitely outclimb my poor diet... I probably helps that I don't eat breakfast though...

It's well known though that as you build muscle, your rest calorie consumption increases, so probably if you build/maintain enough muscle, then you can just outrun your intake, since you consume more without doing anything to start with...

I don't think we're talking about "unhealthy diet" solely in the sense of eating unhealthy foods. The real issue is calorie intake. If you are taking in an unhealthy amount of calories -- regardless of the source -- you are unlikely to be able to outrun that diet.

And you're kinda agreeing with that point: you eat fast food a few times a week, but you don't eat breakfast. So perhaps your caloric intake is still at a reasonable level for your body, regardless of the source of those calories.

I agree. Also age and other activities factor in I'm sure. I suspect you carry your bouldering mat and do some walking to and from boulders.

I think it stands to reason that if we took an overweight person and trained them to eat what you or I eat and then move like you or I move, they'd end up losing weight.

For me though, I know that I can be running say 50km/31 miles a week regularly and that if there is weight loss, it is impercetible to me. But up it with just two more runs and I believe I do start outrunning my diet. Again, this is an n=1 and ignores pretty much every other factor in my life.

I eat fast food multiple times a week

This doesn't mean anything. Just like improving your climbing, what matters is consistency. Count your calories and macros and you'll see that your total intake is reasonable. The processed vs unprocessed argument is negligible when you're only partaking occasionally.

When I was younger, I couldn't understand why I was thinner than my friends. We'd go out to eat and I'd stuff down a huge meal with the best of them. Turns out that they were eating like that at every meal, while I was having cereal for breakfast and a less calorie-dense lunch or dinner.

as you build muscle, your rest calorie consumption increases

The difference is tiny. Yes, you can outrun or out-muscle a bad diet at the extremes, but that's like saying BMI isn't a useful metric. You're not 1970s Schwarzenegger or Phelps.