Addressed in the article - with the same diet, 7% T2D incidence rate in non-affected population, 0% rate for those with the mutation.
I'm sure it happens, but it would be extremely difficult to find people who have similar diets as the Amish, as you'd have to grow your own food and butcher your own animals to achieve a similar level of diet.
Presumable the p-value for normal Amish 7% vs. rest-of-US 8% is not significant in the numbers they studied, but the 7% vs. 0% was.