The bit about Tosca's final B-flat is very real.
It's not just the note; any soprano worthy of the name can hit it ordinarily (the range of the voice goes up to high C and for many beyond). It's that, as Gerry points out in the novel, it comes at the end of a long night's arduous singing. In addition, it's a big leap up from the previous phrase.
Many years ago I had the privilege of hearing the great Renata Tebaldi sing Tosca. She (along with baritone George London) was touring the opera in multiple houses in Europe. Tebaldi and London sang their roles with all the others (including Cavaradossi, the principle tenor role) performed by members of the local company, and the local orchestra playing, conducted by the local conductor. At the performance I saw, at least (in Stuttgart, Germany), it worked out pretty well; it was impressive. Tebaldi was vocally (she was never much of an actress) wonderful in her role, right up to that final B-flat-which she unfortunately missed by the proverbial mile. It was a jarring end to an otherwise terrific performance. It's that kind of thing Gerry's talking about. To this day, half a century later, I still recall that badly flatted note, though I also remember the other fine notes Tebaldi sang that evening.