I'm pretty sure that Diego was 100%, or at least close to it. I think that there are some other factors that contributed to the outcome of that fight.
1) Talent and abilities: Honestly, from the beginning I didn't think Diego stood much of a chance. BJ's boxing is outstanding, and it has been for a very long time, and I heard he's pretty good at Jiu-Jitsu as well, but that's just what I heard.
2) Gameplan: Ok, I got it. GSP was able to push BJ against the cage, wear him out, and take him down. We all saw that. But GSP is a big, strong, athletic welterweight. Let me help out the rest of the LW division that might someday fight BJ Penn. Unless you come into the Octagon at about 180-195 lbs of rip, you're not going to be able to do that to Penn. It didn't work for Florian, and damn sure didn't work for Diego. Stop using that plan, unless of course you're GSP.
3) Experience: This was Diego's first championship fight, whereas it was BJ's 9th fight for a UFC belt, of which he was won 5 fights.
4) Oh yeah, I almost forgot, BJ Penn is a f**king monster at Lightweight.
In summary, an outmatched, less experienced fighter with a piss poor gameplan fighting BJ Penn at Lightweight will probably get destroyed much like Diego did. Can't argue with it. It's just science. Very, very scientific.