While cacao may contain polyphenols that are protective it would be very difficult to get these in sufficient quantity directly from chocolate. There is a small correlation detected in their study with dark chocolate consumption which is not found with milk chocolate. However correlation is not the same as causation. To be fair, the article points out that further randomised controlled trials are needed, which is true. This is exciting research but as always needs to be taken with a pinch of salt and validated in direct mechanistic trials. Please do not start eating 5 bars of dark chocolate a week and think it will decrease your risk of type 2 diabetes. Still the largest modifiable risk factor for diabetes is body fat. Eating multiple bars of chocolate per week makes reducing body fat a bit trickier, although technically still possible if other calorie intake is adjusted accordingly.