I found one article where he’s quoted saying he bought it.
In this day and age in particular, I don’t accept such statements by anyone as fact. I want to see where they got their information. The implication in that simple statement that Hitchcock used it appears to be used as an endorsement of the book by a famous and successful writer. So, no, Hitchcock was not a writer, and whether he actually used the book, liked the book, threw it in the trash – we don’t know. The article I linked to describes how Hitchcock worked with writers – so even his story-telling was a collaboration.
Now, had the statement been that Erle Stanley Gardner has used the book, that’s provable. He did – and he was an actual author.