You would have to ask these folks for their primary literature references, which I suspect won’t go over well.
Feel tired after you eat, e.g. post-meal or postprandial somnolence, is pretty common after high Calorie meals regardless of composition. There does not appear to be an increase in feelings of tiredness with higher carb meals compared to lower carb meals of equal Calorie content. Here Here and more
Outcomes are pretty clear that this claim isn’t overwhelmingly true, which makes the mechanisms less interesting. Looking for a reason why something doesn’t actually happen is a bit silly, you know?
That said, the mechanism behind postprandial somnolence is unknown. There are a number of theories, e.g. higher tryptophan and insulin as you mentioned (more complicated than just carb content), alkaline tide (bicarb moving out into blood stream due to increased acid secretion), and more.
The amino acid theory is not unique either, as it’s been implicated (incorrectly) in hunger. All dietary protein gets broken down into short peptides and single amino acids during digestion. Once in the small intestine, these are both absorbed into the bloodstream of the portal vein, which leads to the liver for further processing and utilization. There’s some that eating higher protein meals would produce higher blood levels of amino acids, which would then increase feelings of fullness.
For example, it was thought that a greater concentration of the amino acid tryptophan in the bloodstream might increase levels of the neurotransmitters serotonin and dopamine, thereby affecting appetite and satiety at the level of the brain. However, there’s no real evidence that eating a higher protein diet does this compared to eating a Calorie-matched meal with lower levels of protein.
Another theory, called the aminostatic theory, predicts that a rise in amino acids within the bloodstream in general will decrease appetite. Veldhorst 2009
However, this theory is not well supported because fasting blood levels of amino acids are not associated with appetite and an increase in amino acids within the bloodstream isn’t consistently associated with satiety either. Moon 2020
Overall, I tend to dismiss anything a keto enthusiast says immediately, as promoting a ketogenic diet as a superior option is incompatible with being intelligent and evidence-based.