I think the condition for weight/fats gain is simply the chronic presence of excess insulin/nutrients.
At a balance level, you will always stay in range.
The key point is that T2D generally have excessive delayed post meal insulin response during the early stages of diagnosis. It is like trying to drive with a loose steering wheel at the amusement park...easy to oversteer. Low carbing helps to minimize the over steering...
For others, they may need additional exercise, less fats, proteins etc depending on their condition.
My anecdotal experience with 18 months of low carb high fats (eggs, butter, roast port belly, nuts, cheese) and intermittent fasting with no exercise (< 1000steps /day) have kept my weight and glucose stable, with fasting glucose averaging around 5.5 mmol...post meal below 7 mmol.
But others may likely achieve the same if they find that the need more carbs to support their daily activity, mental alertnes and require more insulin to maintain glucose stability. They just have to figure out which is an easier and more sustainable path for them...