I wouldn’t say it is total horse manuer. However, one should be aware of a few factors ahead of time before relying solely on pain compliance when defending yourself.
The first is the adrenaline pumping through the body. If flight has not taken place, and fight has, adrenaline can be a great pain killer.
Adrenaline can also be a driving force in a fight. This can also take one way past pain in a fight.
Another factor that comes to mind is the will to survive. People can, and have fought with broken noses, arms, legs, skulls, ribs and well just about anything else that can break, split, or rupture.
Something else to consider is that everybody is different. Pain receptors and nerves are different in people. What could be deemed effective on one individual may completely backfire when tried on another individual due to the sheer fact that people have different body structures in addition to different tolerance levels for pain.
Finally you have to consider drugs and alchohol. If the attacker or person you are defending yourself against is drunk or on drugs, their body may not process pain at all.
The human body and its instinct and will to survive is an amazing thing. Inflicting pain on another person and “waiting” to see if they comply or not should not be your only option.