I’ve heard that theory before. But I don’t think it rings true in the current battlefield environment because they don’t act like traditional military units in how they treat casualties. It’s not an organized atmosphere. If a dude gets shot he continues to fight most of the time if he is decisively engaged. And I personally am not aware of Shia or Sunni fighters going out of their way to help retrieve wounded comrades. My knowledge of how they fight (I’ve never personally been involved in a firefight) is that they more or less scatter like quail when you start applying effective suppressive firepower, and leave their wounds to the individuals concerned.
I agree with you that it doesn’t ring true in actual reality against our current enemy. R. Lee Ermey just put it forward because it was true in his war, Vietnam