Speaking scientifically, muscle is metabolically active tissue, whereas fat isn't. Your body will break down muscle protein and burn that for energy when caloric supply isn't sufficient, especially easily accessible fuel, like simple and/or complex carbs.
All the weight training in the world isn't going to put on muscle if you're not feeding your body the nutrients it needs to build and repair muscle tissue.
As for not being able to out-run a bad diet, people grossly overestimate the amount of calories that are burned in an exercise session. On average, an adult only burns 100 calories running a mile. That's not even enough to burn off a 12oz. Coke. You'd have to run 2 miles just to work off a Snickers bar. Swimming is even less efficient at burning calories, hence the guy referenced who was gaining weight as he got older despite swimming 3-4 hours a day.
And what's even worse as far as swimming goes, the better a swimmer you are, the lower your calorie burn per distance swam since you move through the water much more efficiently than a beginner swimmer. Couple that with the fact that your appetite can get voracious after being in 78-80 degree water for a half-hour or so, and it's really easy to out-eat any caloric deficit you've created.
For personal experience, here was my workout Saturday coupled by caloric intake:
I rode a tick under 60 miles Saturday. I was on the bike for 3 hours.
Pre-ride, I ate a double serving of cereal, about 300 calories.
During the ride, I consumed 2 bottles of Infinit, roughly 600 calories or so.
According to my Garmin, my calorie burn was around 1400 calories, so my caloric deficit was only 500, which I completely used up with the second chicken sandwich I had for lunch.
My BMR (Basal Metabolic Rate) is probably 1500 calories or so, give or take 100 calories, as I'm a small guy. That was probably less than my dinner (2 Big Easy IPAs, fried calamari, cup of gumbo, catfish po-boy, slice of king cake).
Someone who doesn't track caloric intake might be shocked to learn that an intense bike ride only yields around 470 calories burned per hour.
I've tried training on an empty stomach. It doesn't work. At all. I can't hit my interval numbers, I'm sluggish, and as proof of my body catabolizing my muscle tissue, all my workout clothes smell like ammonia after I'm done (byproduct of muscle breakdown during exercise). You NEED to fuel your body to exercise. You need to eat to gain muscle. Period. And while you can't "out run" a bad diet, most people are ignorant as to how quickly your bad diet can wreck any progress you make from training. If they knew, maybe they'd stop with the XL Mocha Latte and muffin the size of their head after their 45 minute Zumba class.
Want further proof? Watch body-builders eat. The Rock has a video or 8 about his eating when he's bulking.
This isn't to say whether or not fasting with do anything with weight loss. But trying to claim muscle mass is a function of exercise and not diet is absolutely ridiculous, and using an endurance athlete as proof is also foolish.