I was in the US ARmy in the 1970s. We first shot at round bulls-eye targets to zero in and familiarize ourselves with weapons. Next, we shot at human shaped targets, (dressed in Asian, VC and NVA uniforms) all in various environments. It was obvious why.
I think in the 1990s Grossman was just reporting the historical transition from bulls-eye to human shape training. Only the most naive police officer and citizen (note that he is not a superstar in military circles) would not know of this common sense, transition and the ideas behind it. He repeated general needs for the how and why of teaching people to kill the enemy. Some citizens are reluctant to hunt animals, least of all kill humans. They need training to do this (and what is new and ground-breaking here?) This info is in any military library for decades.
I think in the late 1990s and early 2000s he had a following amongst non-military-experienced, and somewhat, un-educated police officers.
He was, in the 1990s and still is, a big yawn to me. But the 1990s book chronicled a killing history in a synopsis and therefore is important to own and read and have a working knowledge of.
Hock