That would make sense, IIRC our brains use something around 20% of our caloric intake just at “idle” so it’d make sense to me that they consume more energy when we push them harder.

Do you keep track of what you eat during these tournaments?

I would assume you would have at least a somewhat different diet during these days and your mind would be more focused on the tournament then what you are going to eat, and you might forget to eat enough. It's like going into survival mode when you are focusing on something important to you. Most likely it is a combination of eating less calories and burning more, if you are losing a noticeable amount of weight in a couple days.

According to the article it's not the mental exertion that causes the weight loss, it's the stress and anxiety players experience for extended periods of time, quote from the article: "Grandmasters sustain elevated blood pressure for hours in the range found in competitive marathon runners".

So it's unlikely extended periods of mental exertion would have the same effect without having additional stress or pressure.

My, completely unverified, thought, is that under those conditions of stress and focus, we turn off our digestive system to some extent and literally absorb less calories. It fits well with other factoids I have, like getting sudden stomach trouble under stress.

I boxed for a while. In a match, tension caused significant fatigue above and beyond equivalent time training even if the training was more 'active'. I suspect that's part of it.

I think it's more likely that you eat less during 2 to 3-day tournaments.