Merriam Webster defines dialog as "A written composition in which two or more characters are represented as conversing" so the example you gave isn't dialog; that's for sure. Strictly speaking, it isn't monologue either, so I'll have to retract that. The bottom line for me is no one has yet provided an example of a novel beginning with dialog; I mean dialog as most writers and Merriam Webster understand that term. Just as a reminder, the title of this thread is "starting a novel with dialog."