Type 2 diabetes is not an autoimmune disease. The tendency of people to develop type 2 diabetes rests firmly on human nature.
Junk food is tasty and addictive. Junk food makes people sick. The tendency to get sick from junk food worsens with age. Some people have autoimmune genetics (type 1), some do not (type 2).
Eating junk food makes type 1 people immediately ill. How do they normally respond? They stop eating so much junk food. Eating junk food does not make type 2 people immediately ill. How do they respond? They eat junk food until the secondary effects, the autoimmune conditions, make them ill.
Junk food is not just something which comes in a cheap supermarket box. Junk food can be very expensive. Spending money doesn't wash the junk out of junk food.