As I understand it, the red line is the airspeed at which the rudder can no longer counter the asymmetric thrust in non accelerating flight. In that sense it would be quite accurate and determined by testing (the test pilot ran out of rudder). Note that the plane should be far from stalling, so it wont immediately spin, but directional control is need to reach a desired landing point ...
As a certificated minimum, it would have been determined in the worst possible loading condition I believe.
Cheers