The TKO that happened many many years ago was a perfect strike, right behind the ear. That is very hard to replicate in the heat of the moment. That type of strike is one of the worst you can be hit with because it messes with your equilibrium.
I think GSP has a good chin and mostly anyone hit with that particular strike I mentioned above, would react the same. In fact, a majority of fighters go out cold if hit in that spot.
Anyone remember the matches between BJ? GSP took some serious shots in that first match. Seems people forget and just want to look at the TKO loss, instead of looking at the whole picture.