For high level endurance athletes, eating enough can be a difficult task. I wouldn’t quite categorize diets like the one described in https://www.nytimes.com/2018/02/23/sports/olympics/cross-cou... as “a bad diet”, but it’s certainly a quantity and density of calories that would make it a bad diet for most people with a normal energy expenditure.

An anecdote from my experience with long trail hiking is that essentially everybody loses weight hiking long trails for months. Turns out when you’re hiking 25-30 miles / day, it’s awfully hard to not be in a calorie deficit (especially when you’re also trying to optimize for lightweight food)