FYI, a slight hidden spoiler listed.
So I saw it and actually liked, even though it had Tarantino's expected revisionist history spin on it.
As far as the HM goes. There wouldn't have been a chance to mention it and it wouldn't have made sense. Over 2/3rds of the movie happens months before the night of the Mansion family attack happens, which is only in the last 15 mins of the film. The actual Tate murders happened on the night of August 8th, which is depicted in the film as happening differently but on the same night, and was the day before the soft openings. HM didn't open to the public until August 12th. So even if some in Hollywood might have known about HM soft opening the next day it wouldn't have been something mentioned during a murder scene (depicted as a failed attack next door to Tate in the movie).
So I get that you wanted a HM/Disneyland reference due to the close proximity of the date, but it just wouldn't have made sense given the subject of the film. Even the throw away line referencing Walt Disney was just a topical reference since it was said by an 8 yr old girl. Which does make sense given the 8 yr old girl is suppose to be an "actor" and Walt just died 3 years earlier.