Note that "St. Elizabeth's" is easily confused with Elizabeth, N. J., which was the site of the first Hubbard Dianetic Research Foundation. St. Elizabeth's Hospital is where Hubbard claimed to have spent time as a patient while "crippled and blinded" after WW II. (In fact, I confused the two the first time I read this myself.)
It's quite plausible that he might have addressed the staff at the HDRF in Elizabeth, N. J. -- but would he as a patient have been addressing the staff at St. Elizabeth's Hospital, from a stage? Not likely.
I think Hubbard himself probably confused the two and said "St. Elizabeth's" when he really meant Elizabeth, N. J., or he's just plain lying. And he's probably lying in any case.
Although he was by all accounts an able hypnotist, it's a bit much to swallow that he could hypnotize an entire group of people so thoroughly that they would believe they'd heard a speech when they hadn't.
There are levels of hypnosis. Putting a group into a light trance (relaxation, lethargy, eyes closed) might be doable, but making the entire group think they heard a speech they didn't (positive hallucination) is a different story. That requires a much deeper level of trance.
I think hypnosis plays a large role in scn, but I also think Hubbard is full of shit here.