Good evening Noyade.
"Now tell me, is it really humanly possible for the body to experience 20G, even for an instant?"
In the prone position it might be feasible, since in this position there is no hydrostatic blood pressure difference between head and feet.
However, there could be enormous pressure exerted on the ribcage and abdomen. At the time they obviously thought it possible, but I have my doubts. I have experienced +18G during ejection, but for less than a second, and was fully conscious throughout (although a clean pair of underpants was subsequently required).
LM