I wonder if the muscle attachments from bone to muscle were anchored in different locations compared to modren humans. Muscle to bone leverage could account for a lot of their increased strength.
I think that's true. Humans have pretty tall attachments compared to most animals. Tall gearing is better for endurance running and for throwing.
Neanderthals had knobbier, more robust skeletons. They were more adapted for cold weather and less for running all day across the grasslands. The wave of modern types out of Africa that replaced them was probably better with the throwing spear among its advantages.