Those students could then go home, open a book or the transcript of his lectures and spend 5-10 hours figuring out all the details, and exploring auxiliary stuff not covered in the lecture.
Lecture time is fixed, outside-lecture time is 10X lecture time.
In other words they are lecturing as they are developing the new theory. Clearly such things probably happen much less often now a days, because the overall research landscape is such, that this is not really rewarded. On a smaller scale research seminars in mathematics still are like that though, the results discussed there are often only circulated among the participants, which then make them available to the public piece by piece. Peter Scholzes breakthroughs were widely discussed in Bonn long before they were accessible to the overall public and even then most of what was taught in the initial seminars in Bonn wasn't circulated.
Though some people indeed complicate their lectures for no reason.
In contrast, what students actually need is to transition from one state of mind to another state of mind.
It's still a long way to go, especially for undeveloped countries. However, probably any children born within the next decade will be able to read and speak at least advanced English before reaching 18.
Soapbox UX issue: That's why we language sticklers should tolerate and even welcome the "corruption" of English — that is to say, the crowd-sourced, grass-roots simplification of the tongue. Non-native speakers should feel that it's easy to achieve fluency in English without wondering whether they'll be looked down upon for getting it not quite right.
(That will also help English stay competitive with, say, Mandarin.)
Here's just one example: It should be perfectly fine to say "less errors," vice the supposedly-correct "fewer errors."
More examples?
Edit: I'm not surprised by the downvote, simply bemused.
Though as I recall, von Neumann's family was non-observing. So he was fluent in Ancient Greek by the age of 5, instead. So I'm not that strongly wedded to the hypothesis. Maybe they really were from Mars, as Leo Szilard once quipped.
There was the idea of Esperanto as everyone's second language, so that everyone could talk to everyone with 1/10 of the effort most people now spend learning English, but... it worked about as well as trying to convince people to leave Facebook while everyone else is there.
It's astonishing how much of the web suddenly becomes less broken when you do that...
As American printers were not yet accustomed with the practice of typesetting mathematical symbols, they needed to be taught.
"I heard subsequently that the position that had been offered to me was divided into four pieces"
[0] https://www.maa.org/sites/default/files/pdf/upload_library/2...