Your sauce will still have less sugar than others, but if I understand correctly, simmering for hours will break down the more complex sugars in tomatoes into simpler sugars resulting in a somewhat sweeter taste

I think cooking does also dull the percieved acidity of food though, hence lemon juice or other acids often being added at the end so as to keep the brightness. But I'm not actually sure if the pH changes or if it's just a change in the tartness we associate with acidity, maybe someone can chime in with more information :)