Correlation is not causation. Plenty of people theorize there is large selective advantage here, sure. They may even be correct! Others disagree with them on it being a large advantage.

But even if there is it doesn't mean that there is any reason whatsoever that increased GLP-1 production would be enough of an advantage to have had significant impact on evolution. We're going from A is true, B is likely true, C might maybe be true, all the way to Z is definitely true.