This claim is true, with an important caveat about moderation and amount of use of Dark Chocolate . Dark chocolate at around 70% cacao can offer health benefits for people with diabetes, largely due to its flavonoids found inside. Multiple sources report that individuals who consume at least five servings of dark chocolate per week show a 21% lower risk of developing type 2 diabetes compared to those who rarely eat it at all. Not all chocolate is the same, though—dark chocolate and milk chocolate differ in their effects, even if they have similar calories and saturated fat.
Harvard T.H. Chan School of Public Health notes that dark chocolate’s rich polyphenols may help offset some of the impacts of sugar and fat on weight gain and diabetes risk, whereas milk chocolate can have negative effects when eaten in excess. Studies also indicate other potential benefits of dark chocolate, including positive effects on arteries, metabolism, and cardiovascular health. It’s worth emphasizing that participants in these studies typically consumed small amounts, so the findings don’t extend to large portions. Overall, the research points to real benefits from moderate dark chocolate intake, while consistently cautioning against high consumption.
 
Sources : https://pmc.ncbi.nlm.nih.gov/articles/PMC4307206/ 
Eating dark chocolate linked with reduced risk of type 2 diabetes | Harvard T.H. Chan School of Public Health