Ketones are associated with fasted states and/or restricted carbohydrates. The average person holds about 400-500g of glucose (1600-2000 calories) in their bodies. Once that is depleted, through fasting or carb restriction, you will switch to ketogenesis to provide fuel. There are numerous studies that show a positive correlation with ketone production and cognitive performance.
Few examples: > https://pubmed.ncbi.nlm.nih.gov/25404320/ > https://pubmed.ncbi.nlm.nih.gov/27528626/
If I am doing serious deep work, I can go 8 to 10 or so hours without even thinking about food. However, once I do eat, I'm done with any serious work for a few hours. Keep in mind, I'm operating from a 20 year experience of intermittent fasting (before it was cool).
To reiterate, I don't think it's a delusion. We evolved in a feast or famine state. Dense carbohydrate sources and constant satiation were rare when our base metabolic pathways evolved.