8lbs if you're already severely overweight or loaded on carbs, maybe. These guys don't exactly look like they are doing either of that. To put it in a different perspective, that amount of weight loss beats out strict waterfasting at the same weight. You can't get much more strict than straight-up dehydrating (because we all know dehydrating yourself prior to a mental exercise is a great idea) or doing rigorous exercise outside of that.
And no, chess is not rigorous exercise. If it was, the room would be too hot for them to wear suits and tuxedos. Those burned cals have to go somewhere. For the unconvinced, try jogging or lifting 2000 calories worth in a suit at room temperature or slightly above without taking it off.
I bet food poisoning would do it. Because, like you said, they aren't burning those calories - 6,000 calories in a single chess game would leave them in a giant pool of sweat.
And no, chess is not rigorous exercise. If it was, the room would be too hot for them to wear suits and tuxedos. Those burned cals have to go somewhere. For the unconvinced, try jogging or lifting 2000 calories worth in a suit at room temperature or slightly above without taking it off.