Their conclusion makes sense to me.

Since diabetes is likely caused by obesity in many cases the lack of diabetes may be a direct or indirect result of the exercise.

Obesity is often the result of lack of exercise to a large extent.

Exercise is at least one of the major contributors to good heart health.

Better diet, quitting or not starting smoking, and moderation in alcohol consumption all tend to go hand in hand with regular exercise. Even if they may not be the direct result of it they are likely to be an indirect result of it even if only due to increased sense of well being.