Thought I'd venture a reply, vs starting a new thread that would have inevitable redundancies.
Its a no brainer Taratinio is doing alternate history here. He's done it overtly, and covertly is at least two of his movies. I suspect if one looks theres alternate time line elements in others. In O...in Hollywood the PoD of team manson stumbling into the wrong address & into the wrong people is plausible. OTL they were looking for the owner of the residence & not expecting tenants. So Taritinos story line is simply a slightly different divergence there. Running into people capable of kicking team mansons ass is not exactly ASB either. Hollywood never has been & is still not entirely people incapable of defending themselves. A off duty cop working as a celebrities body guard would be not more implausible than a slightly drunk and pissed off stuntman. ASB territory would have been Charlies Deamons running into Bills Angels and the story ending with Lucy Liu, Uma Thurman & the rest staring at the broken corpses on the floor and asking each other "Who the f..k was that we just wasted". Cute but not as satisfying as Brad Pits stuntman effort.
The consensus seems to be Taratino did alternate history well here. I enjoyed this above most of his movies & the twist/s at the ending were a large part of that. I suspect Taratino put a bit more AltHist in this than just the surprise ending. But. Im not the expert on late 1960s Hollywood or the Manson Family, so that is left for others. Hope someone else can weigh in on that aspect.