what? Definition of a flat spin is that there is no forward movement at all, while, at the same time, there is a great rotational movement. How do you define flat spin?
That is NOT a definition of a flat spin. A flat spin is one where the attitude is close to the horizontal plane. Forward motion doesn't enter into it. It is entirely possible to have a flat spin with some forward motion.
Please stop posting comments that extend far beyond your knowledge.