This claim is true, the article supports this claim well. The NPR article highlights how dark chocolate can help improve insulin sensitivity, insulin being a hormone that regulates blood sugar. Dark chocolate has epicatechins, which are known for their antioxidant properties, which, according to researcher Jeffery Tessem, are good for those who suffer from Type 2 diabetes. The article also mentioned how those who ate milk chocolate experienced no change in diabetes risk and, in many cases, gained more weight. The National Library of Medicine has also done significant research on Dark Chocolate and Diabetes, They explained how Flavonoids, a component in the cocoa plant, may improve insulin resistance by reducing oxidative stress, which is the main offender in insulin resistance, i.e., diabetes. This claim is reliable.
https://pmc.ncbi.nlm.nih.gov/articles/PMC5699188/