Three kinds of muffins were on the menu: a high-fat, low-sugar muffin
...wait what? A high-fat muffin?
If the study was looking to determine which diet was the "best diet" based on postprandial glucose readings, then an omnivorous ketogenic diet would produce similar results across the entire cohort and maximize this supposed healthfulness metric. I would imagine that the postprandial glucose variation between twins eating something like eggs and macadamia nuts for breakfast would be minimal.
Interestingly, looking at their PREDICT 2 study (https://predict.study
), they're offering to help participants find their optimal diets... while testing individuals on a selection of vegetarian foods, which may include dairy, gluten, refined carbohydrates, and sugar.