And the most glaringly obvious - bouncy-shoes. Remember folks, when everyone is special, no one is. That time would barely crack the top 50 from last year. In 2019, a year before bouncy track spikes, it would have been top 20.
I know someone who went from 4:29 to 4:17 in 10 months. He also managed to eek out a sub 4, 5 years after the 4:29. In the first 10 months he grew 3 inches and was getting much better training and nutrition. Didn't hurt to be on a ncaa xc team that had 10 guys fighting for 7 spots every year. Nor the fact that he trained occassionally with an Olympic Champion teammate. 800 was his favorite race where he achieved higher performance but it was the strength training over time that was the key factor of consistent improvement. Had a couple injuries along the way that were managed well but still setbacks to more consistency.
Genetics, Environment, The Plan, Motivation. Genetics was his weakness. Had to make up for it in the other 3 as much as possible.
All this pre super shoes, bicarb, etc.... Times have changed. If he was in his prime now, not sure anything slower than 3:55 would mean that much. But he might have been at 3:53 which is still a mind blowing effort.
How fast is the herd moving? There's always going to be a few barrier breakers, a chase pack, the pelaton and the stragglers waiting for the broom wagon ride home. Over time, the herd just keeps moving faster.
Focus on the competition in the moment and enjoy it for what it is. Historical reference is OK but there have always been defining lines separating eras. Nothing matters in the moment of truth in a hot championship race but the truth of what is possible in each person's mind.