It seems that at the distances they were testing for there was no consequential loss of velocity. This is because the spears picked up some speed from gravitati

It seems that at the distances they were testing for there was no consequential loss of velocity. This is because the spears picked up some speed from gravitational acceleration, due to being released at a greater height than the target. Since the spears are heavier and have much less drag compared to projectiles like arrows that are fletched, it seems like drag losses and potential energy transfers more or less cancel out. In fact for some throws impact velocity was actually greater than release velocity.