"Even so, cavilling about achieving the exact position seems a bit much, the point is to make yourself as small as possible and protect your head,"
It's also about stopping you breaking your neck, which is what happened to some unfortunates in otherwise survivable accidents who adopted the previously recommended brace position.
And perhaps more importantly, in a survivable accident, it is all about protecting your legs. Again, people have perished, because they either broke their shins on the seat in front, so becoming immobilised, or breaking/trapping legs under their own seats. If you look at any safety card, the legs are drawn in a very specific position