Rule for questions like "Or is it?"
Your example is an instance of the rhetorical figure called metanoia: Making a statement and then correcting or questioning it so as to possibly refute it.
Examples:
If she come in, she'll surely speak to my wife / My wife, my wife! What wife? — Shakespeare, Othello, Act 5, Scene 2
No other should taste the happiness I scorn. Why do I say happiness? — Fielding, Joseph Andrews