I do not see any inconsistency with what you say. Our early upright ancestors were not all that bright, and the enlargement of the brain coincided with other changes. I imagine Lucy's greatest skill was being able to run, chase down game and whack it with a club. Australopithicus and Pithecanthropus were beginning to make tools, if I recall correctly.
An aboreal dweller on the ground loses most of its survival skills. We compensated by walking upright, wielding clubs and rocks, and living and working in groups. A man with a club is no match for a lion; but 10 men with clubs and spears are. We also learned to cook and eat meat, which gave the calories needed for the larger brain. Also, a human hand, designed to do delicate tasks, is not the same as one designed for a brachiating animal.
We have rearranged our environment like no other being. I have never seen a Chimp eating Omaha steaks, living in a codo and driving a cadillac wearing a Brooks Brothers suit, and running a corporation.