When the only tool in the box is a hammer...

Everything that needs fixing looks like a nail.

People with diabetes mellitus are issued with blood glucose meters - and nothing else.

For people with type 1 diabetes, that's fine. They lack insulin, so they have to inject insulin in the right amounts & types to keep their blood glucose levels within reasonable limits. Applying Bernstein's Law of small numbers by reducing glycaemic load to a minimum keeps blood glucose levels within reasonable limits (between 3 & 7mmol/L) most of the time. See also The problem with Diabetes.

For people with type 2 diabetes and excessive visceral (belly) fat (~85% of people with type 2 diabetes), that's not fine. Their disease is a disease of chronic excess intake relative to oxidation, causing fasting dyseverythingaemia
(hyperglycaemia, hypercholesterolaemia, hyperNEFAaemia, hypertriglyceridaemia, hyperuricaemia, etc). People who have type 2 diabetes don't have only postprandial hyperglycaemia - they also have postprandial hypertriglyceridaemia. See Lifestyle Intervention Leading to Moderate Weight Loss Normalizes Postprandial Triacylglycerolemia Despite Persisting Obesity. Postprandial hypertriglyceridaemia is atherogenic. See Ultra-high-fat (~80%) diets: The good, the bad and the ugly.

However, because the only tool in the box of someone with type 2 diabetes is a blood glucose meter, their disease looks like one of only hyperglycaemia. Applying Bernstein's Law of small numbers by reducing carbohydrate intake to a minimum keeps blood glucose levels within reasonable limits, but makes other things worse if energy from carbohydrates is replaced by energy from fats.

Only if energy from carbohydrates is reduced AND energy from fats isn't increased to compensate (i.e. eat a LCLF PSMF or Modified PSMF until sufficient visceral fat has been lost), does carbohydrate restriction help people with type 2 diabetes.

Dr. Richard K Bernstein on insulin for type 2 diabetics, and some definitions.

Dr. Richard K Bernstein:-

Dr. Richard Bernstein, the world's leading low-carb diabetologist, says in Diabetes: The Basics:-
"Many people (including the parents of diabetic children) view having to use insulin as a last straw, a final admission that they are (or their child is) a diabetic and seriously ill. Therefore they will try anything else - including things that will burn out their remaining beta cells - before using insulin. Many people in our culture have the notion that you cannot be well if you are using medication. This is nonsense, but some patients are so convinced that they must do things the “natural” way that I practically have to beg them to use insulin, which is as “natural” as one can go. In reality, nothing could be more natural. Diabetics who still have beta cell function left may well be carrying their own cure around with them - provided they don’t burn it out with high blood sugars and the refusal to use insulin."

Some definitions:-

From Low-carbohydrate diet:-
"The term "low-carbohydrate diet" is generally applied to diets that restrict carbohydrates to less than 20% of caloric intake, but can also refer to diets that simply restrict or limit carbohydrates."
A typical woman consumes ~2000kcals/day. A typical man consumes ~2,500kcals/day. Therefore...

Very-low-carb diet = <10% energy from carbs ≡ <~50 or ~62.5g carbs/day.
Low-carb diet = <20% energy from carbs ≡ <~100 or ~125g carbs/day.
"Healthy eating" = >55% energy from carbs ≡ >~275 or ~344g carbs/day.

The problem with Diabetes.

Referring to Blood Glucose, Insulin & Diabetes, a healthy person regulates his/her blood glucose level so that it doesn't go too high or too low.

Someone who has diabetes either has insufficient/no insulin secretion (type 1) or has insulin secretion but it's ineffective (type 2). People with type 1 diabetes have to inject various types of insulin and monitor their Blood Glucose (BG) with a BG meter. Here lies the problem. Consider the following analogy:-

Blood glucose control is like driving down a road cut into the side of a mountain. If you steer too far to the left (low blood glucose), you fall off the edge of the road and die. If you steer too far to the right (high blood glucose), you smash yourself against the side of the mountain and damage yourself. Injecting insulin/taking oral hypoglycaemic agents is like pulling on the steering wheel to the left. Eating sugary/starchy carbs is like pulling on the steering wheel to the right. Exercising is like pulling on the steering wheel to the left.

Not monitoring BG regularly is like driving down the above road with your eyes shut most of the time. You have no idea where you are on the road. Every time you monitor BG, you open your eyes.

A high-glycaemic load diet + high-medication regimen (e.g. ADA & Diabetes-UK) is like pulling on the steering wheel hard, which results in diabetics veering all over the road. As diabetics don't want to fall off the edge of the road and die, they steer too far to the right (high average blood glucose) and damage themselves due to glycation.

A low-glycaemic load diet + low-medication regimen (e.g. Bernstein, Christie, Ruhl, Cooksey, Shwarzbein etc) is like pulling on the steering wheel gently, which results in diabetics steering a reasonably straight course down the middle of the road. Regular BG monitoring allows rapid error-correction.

As low blood glucose is potentially fatal, that's why Studies show that a high average blood glucose level has lower mortality than a more normal average blood glucose level.