The key is word Whole, a whole food plant-based diet is the only diet documented to reverse the progression of heart disease.
The reversal of heart disease with this diet has been proven decades ago - https://nutritionfacts.org/2011/10/14/heart-disease-there-is...
No one knows the exact cause of diabetes, there could several causes. Processed foods are not whole foods. White flour, marguerine, processed sugar, cheetoes, that's a not a whole food it's not comparable to potatoes and bananas.
Here is the opinion of one of the leading experts in Diabetes, Neal Barnard - https://www.youtube.com/watch?v=S51D07bvlPY
He mentions that diabetes seems to be linked to an excessive consumption of fat, that inhibits the absortion of sugar.
He has studies published showing that a whole-food plant based diets in some cases has made the person stop taking insuline altogether.