Diabulimia is another one of those eating disorders that is not listed in the DSM (Diagnostic and Statistical Manual of Mental Illnesses). So while it is true there is no diagnostic criteria for this ED, that doesn't make it any less of one ... and it doesn't make it any less lethal.
The relationship to Diabetes
Diabetics give themselves insulin. With this eating disorder, the person will intentionally restrict the insulin they need. The result is the body goes into a state of starvation. Once in starvation mode, any sugar that the person eats (and sugar is in most foods - just not necessarily in its raw form) doesn't get used properly. Sugar is what gives us energy. Any excess amounts that aren't needed for energy are stored as fat for later use.
Using insulin for rapid weight loss
With insulin restriction, the sugar that comes into the body is immediately passed out in the urine. The sugar isn't stored as fat, nor is it used for energy. This leads to rapid weight loss.
Typically someone who becomes a diabulimic has already been diagnosed with bulimia nervosa or hasanorexia symptoms. Controlling the insulin injections is simply a new way to control and lose weight. In the diabetic community, disordered eating behavior and disturbed eating behavior are widely recognized and have been for years. Putting the label on this behavior is simply a way to categorize the intentional manipulation of insulin for weight loss.