To be honest, I can believe this. I once split my head open in a diving accident... and while I felt foolish at the time, I had no clue I was open and bleeding 'til someone told me to get out of the water. I pulled myself out... they made me lay down... and I was still not feeling any discomfort. It wasn't until someone tried to stem the bleeding with ice that I felt any pain what-so-ever. But trust me, it sure did hurt then! That ice darn near sent me flying!
The docs at the hospital said that sudden, severe injury can sometimes induce a "rush" of endorphins... and the traumatized patient will not feel anything for a short while afterward.