While this claim may be true, it is not clearly supported by the study referenced in the Medscape article linked. This study specifically researched groups that ate one ounce of dark chocolate daily and those that ate one ounce of milk chocolate daily, coming to the conclusion that consumption of dark chocolate lowers risk of type 2 diabetes compared to consumption of milk chocolate rather than no consumption of chocolate. Thus, it cannot be concluded solely from this study that consumption of dark chocolate leads to lower risk of type 2 diabetes, nor can consumption of dark chocolate be recommended to readers in order to reduce risk of type 2 diabetes. However, the study does include significant scientific backing with discussion of flavanols present in cocoa that reduce blood pressure, making the claim that dark chocolate lowers risk of type 2 diabetes worth researching.
https://www.medscape.com/viewarticle/dark-chocolate-bittersweet-remedy-diabetes-risk-2024a1000mg2?form=fpf
However, studies do show that consuming chocolate will lead to lower risk of cardiovascular disease, stroke, adverse outcomes, and cardiovascular mortality, all of which are at increased risk for those with type 2 diabetes. A meta analysis consisting of nine studies and 157,809 participants shows that those who consume 100g or about 3.5 ounces of dark or milk chocolate, with more participants consuming dark chocolate, was associated with lower risk of cardiovascular disease, stroke, adverse outcomes, and cardiovascular mortality.
https://heart.bmj.com/content/101/16/1279.short?g=w_heart_ahead_tab
Thus, the claim that dark chocolate lowers risk of type 2 diabetes may be true, but requires further research in relation to those who do not consume chocolate rather than those who consume different types.