I think he overstates the effectiveness of Minie rifles. Only specialist troops could estimate ranges accurately enough to attain serious results at extreme range. In the hands of ordinary troops they achieved results at moderate increases in range over smooth bore muskets. But since the Russians were deployed in massed columns and had cartridges wrapped in inferior paper, they were a great target and didn't generate withering return fire.
Moderate range increases against packed targets and middling return fire explain the heavy Russian losses in the open field.