As a type 2 diabetic, i’m not sure they even know with a high degree of certainty what it is. They know symptoms and can do some measuring that provides numbers. They can then correlate those numbers with various maladies.
I can testify that weight loss of about 15% and transition from a obese to overweight biomass yields decrease in various critical numbers. Those numbers already controlled to “normal” by drugs were reduced further by the diet and exercise. Diet meaning reduce carbs. Exercise meaning do something physical to raise your heart rate every day.
A primary difference is the elimination or serious reduction in acid indigestion. Sleep patterns change, more sleep, lest wakefulness at night.
The malfunction is complex and I believe related to all the various numbers being high not just blood sugar/A1c. Although still on the high end of over weight, I feel much better when I don’t eat too many carbs and get some vigorous exercise. The serious exercise I now face is raking leaves probably till the end of January
I think you are right. Just saw this article about type 3 diabetes(maybe) they are not sure yet.
The idea that Alzheimers might be Type 3 diabetes has been around since 2005, but the connection between poor diet and Alzheimers is becoming more convincing, as summarized in a cover story in New Scientist entitled Food for Thought: What You Eat May Be Killing Your Brain. (The graphic a chocolate brain with a huge piece missing is creepy. But for the record: chocolate is not the enemy.)
The studies  are increasingly persuasive, and unsurprising when you understand the role of insulin in the body. So, a brief lesson.
We all need insulin: in non-diabetics, its released to help cells take in the blood sugar (glucose) they need for energy. But the cells can hold only so much; excess sugar is first stored as glycogen, and when theres enough of that as fat. (Blood sugar doesnt come only from sugar, but from carbohydrates of all kinds; easily digested carbohydrates flood the bloodstream with sugar.) Insulin not only keeps the blood vessels that supply the brain healthy, it also encourages the brains neurons to absorb glucose, and allows those neurons to change and become stronger. Low insulin levels in the brain mean reduced brain function.