What will cure/prevent type 2 diabetes is a low-carb, high-fat diet. Sardines are a low-carb, high-fat food. Eating them replaces a big load of the carbs people usually consume, and just like that they reduce the amount of insulin they have to produce, reducing their insulin resistance and so on. Omega-3 fatty acids are just a slight benefit, probably, and not so much for the diabetes. But "Just eat a can of sardines…
Strength training, and good sleep to support the training, also help a great deal, but is not as easily managed as adding something relatively tasty and convenient to one's diet.
Eating a high quantity of diverse vegetables every day is a great idea but the practical effectiveness stumbles due to the difficulty of the habit change.