It is a spin when one wing is stalled more than the other. The rotation results from the less-stalled wing providing more lift than the other.
A stall occurs when both wings stop flying about equally. Recovery occurs if the cg is ahead of the C/L, center of lift, as the nose will drop, and flying speed is regained.
If the cg is behind the C/L of stalled wings, the nose will not drop, and the plane will not regain flying speed. This is a flat spin. It does not require rotation. As above, for rotation there has to be a partly unstalled wing. Per Wiki, the flat spin angle of attack will be between 65 and 90 degrees, which means there could be some forward speed.
Modern airliners are intentionally flown at aft cg near the safe limit. A load shift will put the cg behind the C/L, and this has happened countless times, usually with cargo.
GB