I was trying to copy/paste some of the relevant parts for you on the macronutrient discussion but honestly there’s a LOT there and if you’re interested, it’s worth the read. If you scroll down to the Discussion portion of the paper, then scroll down a few more paragraphs, it goes into some detail about their mechanisms. Most notably I think it the distinction between glucose bs fructose, and also the overall macronutrient breakdown of the meal.

“First, unlike the OGTT, the High Carb breakfast a contained 23% proportion of other macronutrients, of which 16% was fat and 7% was protein. The ratio of carbohydrates-to-protein is known to modulate tryptophan availability in the brain and thus serotonin synthesis26. Specifically, pure carbohydrate (i.e., a ratio of 1:0, as seen in the OGTT) may drive the strongest sedative effect through a surge in serotonin synthesis and thus brain-available tryptophan associated with impaired alertness27. Our finding would therefore suggest that the combination of other macronutrients (e.g., protein, fat, fibre) paired with carbohydrate determines the true synergistic impact of the meal on subsequent alertness.”

Anyway, the discussion is really interesting tbh and worth the read.