I think it's more reasonable to call it a plateau than a ceiling. There's only so much energy the body can conserve in other activities.

If you do six hours of exercise burning 1000 calories per hour, it's physically impossible to compensate that away: in that day you burned at least 6000 calories, and more realistically upwards of 7000 (keeping your body alive for another 18 hours, building/repairing muscles, etc). I guess the article conveniently leaves that out because it's largely irrelevant: few people are able to invest that amount of time into their diet.