The atlatl darts used in this research were much lighter than the spears, as is usual for atlatl darts. So you can carry more of them. Also, atlatl darts are faster and more accurate - the difference demonstrated here is of energy on impact, which is more important when hunting big game - and of relative differences in performance when elevated vs. even ground.

As I understand it, the atlatl enabled much more efficient and powerful throwing without much loss of accuracy. A brief search for figures says it can hit a grapefruit at 40m range

"For an experienced atlatl thrower, the mechanical advantage of the atlatl over throwing with the arm alone is about 6:1. Thus, if you can throw a dart 15 meters with your arm alone, you could throw it about 91 meters with the atlatl."