It's a perfectly reasonable measuring stick for a number of reasons;
1. The two fights were back-to-back (although 1 year apart due to his suspension). It's tough to compare a non-roided 32 year old AO, to a mid-20s (likely roided up) PRIDE version.
2. Looking back - Brock's health certainly raises some eyebrows and questions.
3. The obvious factor that AO was on roids vs. Brock, and wasn't for this fight.
The book is essentially closed for AO for me. Can he handle some mid-range heavyweights? Probably, but I don't see him living up to the hype as a contender for the belt.
I respectfully disagree. People saying, 'Well, he fought like a raging bull against Brock and was a bit sluggish against Silva, so he must have nothing left' are mistaken. This same scenario played out in SF. He fought like a raging bull against Rogers and then put on an absolutely terrible, slow performance against Fabricio. The point is that Overeem has never had one speed. He's been slow and plodding. And he's been fast and deadly. In that respect, the Brock fight is not 'the' measuring stick. Basing a man's future on one fight is silly to begin with, especially when he was winning said fight, but to each their own.