There are a few famous instances where mathematicians ran seminars for close to a decade and developed extensive theories on that way. Von Neumann appears to have been one, Grothendieck is a famous other example. The lectures are basically then meant to be taken by people that dedicate almost all their time to understand them.
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.
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.