You had me until the very last line.
What makes a PB so special is that it combines all the factors, wind, weather, fitness, competition, pace, etc and is, by definition, your personally best race ever. Particularly for a runner like Kipyegon who has run dozens and dozens of timed 1500s. She's never gone under 3:49 in the 1500, which isn't sub-4 (or even sub 4:05) pace.
People consider all the benefits that an experiment like this might bring, like pacing or fueling or shoes, and those combined I agree probably would beat your PB on aggregate. However, the issue with these single event trials is that you can't in fact control these factors. You can't control the temperature, allegedly it was a bit hotter than preferred today, you can't control the wind, the humidity, you can try to impact fitness and mental state but you can't guarantee that your athlete will be at their peak fitness either. While it's possible your trial event has the best possible conditions, it's highly statistically unlikely.
The other thing that is flawed about this experiment is that this runner who saw 3% gains, presumably ran on the same track. Possibly even on the same day with virtually the same weather. Likely close in time, so her fitness would be the same. It doesn't compare the time to the same runner's PB in the distance, which would likely cut into the 3% gain significantly.
If Kipyegon ran dozens of time trials over the span of say one year, eventually she would get a weather day and fitness that was as favorable as her 3:49, and then maybe you could expect something approximating a 3% gain in time on top of that, although I agree that a 4:30 runner likely has more room for growth than a 4:07 runner so maybe slightly less than 3%. But that kind of experiment would be A. very expensive and B. very time consuming for the athlete, who might be interested in running...any other race that year.
If you just run it back just one more time, there's no guarantee that her fitness is better, the weather is better, other conditions are better, etc.