Peak vs Prime is a matter of definition and time.
An athlete's Prime is over a period of time when they are consistently very good or close to their best, over that course of time. Usually measured in years.
The peak is the single day that they are at their absolute best.
With sports that are quantifiable or measurable, like timed sports (sprinting) or strength-based sports (weight lifting) the peak is when the athlete produces a PB they never achieve again.
Prime though is the plateau of measurement on either side of that PB where they are very close to this peak.
Usain Bolt's PB is 9.58 in 2009 that is his PEAK.
However, he was in his Prime where he was on top of the world from 2008-2016. So his Prime lasted 7-9 years.
2008 9.69 sec
2009 9.58 sec
2012 9.63 sec
2013 9.77 sec
2016 9.81 sec
In fighting sports it's harder to get a definitive measure b/c there is no yard stick-like in other sports to quantify performance so it's more of an objective determination based on performance.
GSP was in his early Prime in the Serra fight but he had not yet peaked.