I understand what you are saying re the stability triangle, and the fact that when the forks are extended, the center of gravity moves closer to the front, making the truck more stable, left to right.

For the mindset that believe that the narrow aisle reach truck has a stability trapezoid instead of the triangle, obviously the lateral space is wider than the triangle at the back of the truck, allowing for the truck to be almost, but not quite as, stable when the truck is empty, then when is full.

Therefore, if the truck has a triangle, and one is maintaining the forks in their nested position, the truck would seem to be more unstable, understandably.

If its confusing, then I can paint you a picture, and I would need instructions on how to send it to you, or you can drop the matter as far as my comments are concerned, and just leave it at that! lol