I totally agree: it's completely preposterous that they're burning 6kcal sitting still regardless of how much they're thinking. People don't seem to realize how hard it is to burn that many calories when moving. That is advanced athlete levels of caloric burn. Does this caloric burn similarly apply while they're "training", or just at tournaments? Do they need to put down Olympic level meals every day just to sustain?
What about other people who's job is thinking? Are mathematicians also forced to devote a significant part of their day just to ensuring can chew through 6000 calories worth of food? Or is there something about board movements that consumes extra calories? L shape? 3 calories. Straight line: one calories. If we could pin down which type of thought burned 100x more calories, we'd be onto something powerful here.