This is an interesting analysis. I see a number of factors in play:
1. Population size: the more people you have, the more likely you are to make "geniuses". The genius chart late in the piece maps with this hypothesis;
2. Baseline education: the idea here is that geniuses are less of a gap if the normal level of education and competency is higher;
3. Low-hanging fruit: things seem obvious in hindsight of course but it's also true that some of the big jumps in certain fields come down to what were fairly simple ideas. Those who come up with them are typically labelled "geniuses". That may or may not be the case. But the point is that progress in fields isn't smooth. We've now been in a period in physics where for decades now we've simply confirmed what we already suspected. Useful of course. As is disproving various theories (which is constantly happening).
But the 20th century had 2 massive jumps forward in physics: namely relativity (obviously) and the various quantum mechanics related fields (QFT, QCD, etc). This isn't my area of expertise but my understanding is that a big part of this was realizing just how deeply tied physics and certain areas of mathematics are.
Oh and for the record, I'm really talking about fundamental physics here. Other fields like condensed matter physics are a completely different beast.
But is the 20th century typical? It's hard to say. I suspect it isn't. I once heard research described as spending years of your life working on a problem and your reward is you get to throw a few pebbles on a pile. Eventually that pebble pile becomes a mountain. Someone throwing more than a few pebbles on is realtively inrequent.
I'm not sure how much "aristocratic teaching" really has to do with it.