I'm afraid my reaction was that it's as much obviously nonsense as the concept of the Hidden Plot in the movie -- they even admit openly that you have to change the order of events and to ignore real-life clinical definitions in order to produce this interpretation. It's not a serious suggestion about the events of the musical, just a way of engaging medical students by tapping into popular culture (see the list of other stories used in the course, and how those are being interpreted).

I mean, it's a bit tough on the staff and management of the Opera House to learn that the death and destruction of the last few months was all supposedly carried out by a figment of a depressed chorus girl's imagination ;-p
I don't think there's any doubt that the events both of the book and of the musical are being presented as really having happened; Leroux in particular makes a speciality of plots that feature supposedly supernatural happenings with a prosaic explanation, and the Phantom's activities affect even those who refuse to believe in him.

It just amused me to see a straight-faced piece of highly improbable analysis that for once took it for granted that Christine's relationship with Raoul was real, and to be encouraged, and that her entire relationship with the Phantom was an unhealthy delusion :-p