Over stretching is NOT a myth. There are a couple of different ways to look at this.
First is over stretching causing microtrauma during the stretch due to forcing a stretch too hard, too far, and too fast. Possibly contributing to the cumulative trauma cycle and causing compensation patterns due to the body’s protective mechanisms.
Second is the length tension principle. There is a point in a muscles length when it is the strongest due to optimal overlapping of the actin and myosin. If a muscle is lengthened to an extreme there will most likely be strength deficits at certain points openning that person up to an injury. It is possible to strengthen extreme ranges of motion and very important if a person is hyperflexible.