Mozart didn't hear it "as it were, all at once"?

"I had no idea, until I read Peter Hacker's review of my book Selves: an essay in revisionary metaphysics (January 22), that the letter in which Mozart supposedly claimed he could hear 'as it were, all at once' the whole of a piece of music that he was composing in his mind was a 'notorious forgery'.  I found the letter quoted (in all innocence) by William James in his Principles of Psychology (1890); it turns out that it was invented by the critic Friedrich Rochlitz (1769-1842).  How could anyone do such a thing?  The fact that Goethe was duped (and very moved) by the letter doesn't make it any better.  The fact that Rochlitz knew Mozart, and presumably wouldn't have wanted entirely to misrepresent his views, doesn't make it much better.
"Rochlitz's mock Mozart describes a point in the process of composition when the piece 'is actually almost finished in my mind, so that even if it is a long piece I can see it as a whole in my mind in a glance, as if it were a beautiful painting. . . . In my imagination I do not hear the parts successively, as they must come out, but, as it were, all at once'."

Galen Strawson, "One's got it" ("Letters to the Editor"), Times literary supplement, February 5, 2010, 6.  Strawson goes on to defend, though, the use he had made of this forgery in his book, i.e. "the view that this corresponds to something experientially real, especially in musical genius."