I'd guess that what is happening is that he was drinking diet soda sweetened with aspartame, which may cause the body to generate the same insulin response as a drink sweetened with real sugar Insulin facilitates the metabolism of carbohydrates. Diet Coke, with aspartame, contains 0% carbohydrates. It provides 0 calories. Pretty hard to stimulate insulin without calories.
That's the point - even though aspartame does not contain carbohydrates, it has been claimed to stimulate an insulin response, which can lead to metabolic problems, and which has led many type 2 diabetics as well as people who believe that they may be at risk for diabetes to substitute sweeteners such as Splenda for aspartame, in the belief that such response is either less or nonexistant with Splenda.
Is it true? Do a google search for articles on "cephalic phase of insulin release", for example here. There are also studies that would seem to dispute such claims, with respect to aspartame (for example, see this one).