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.
In my view the post-surgury Brock loses to even a non-steroided Overeem. I also believe the pre-illness/surgury Brock beats even a roided up younger Overeem.