I believe the police and maybe the Asst. DA who wrote that book covered up a lot of what actually went down. What is established fact is that Manson was familiar with that neighborhood as he spent time in the house next door. Rumor was that Leno LB was heavily in gambling debt to the maf eye A, and Rosemary was dealing LSD. Lots of dope burns were going on at that time and it's rumored that the Tate killings were a result of a drug turf war.
To me, that is more believable than random slaughter. I am much more inclined that Manson, who was established to be a nutter, would be a very useful tool for whoever wanted to settle scores.