The definition of a scrimmage kick requires there be a "punt, drop kick, or place kick". These kicks are defined in Rule 2-24 as to their exact nature. Since the kicking of a muffed ball does not comply with the definition of a place kick, punt, or drop kick, it can not be called a "scrimmage kick" and therefore IMO would not fall into the "tack-on" category. The ball is treated as a muffed ball and R can take the ball at the OOB spot, or replay the down following a penalty on K.