Envelope = cover?

Hello Friends,

today I would like to ask, whether it's possible to call a book cover "an envelope" - like in the Harold Bloom's column:

"What's happening is part of a phenomenon I wrote about a couple of years ago when I was asked to comment on Rowling. I went to the Yale University bookstore and bought and read a copy of "Harry Potter and the Sorcerer's Stone." I suffered a great deal in the process. The writing was dreadful; the book was terrible. As I read, I noticed that every time a character went for a walk, the author wrote instead that the character "stretched his legs." I began marking on the back of an envelope every time that phrase was repeated. I stopped only after I had marked the envelope several dozen times. I was incredulous. Rowling's mind is so governed by cliches and dead metaphors that she has no other style of writing."
If it's not about the cover, do you think he simply engaged some random letter envelope that happened to lie on his desk, or is it yet some different object?

Marta