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.
I sometimes do it, when I am angry at a person, not on par with me intellectual. Then out of bad old habit I speak a very high language to underline my competence and lower the other person for not understanding it. I witnessed that behavior quite some times with other persons, too and thats probably why I aquired it myself at some point. But I try to get rid of it, as it is not at all helping with communication. And I think the high lecturing is in the same category. To boost oneself.
But agreed, most too high lectures are probably just happening out of lazyness or unawareness, to get down on the level of the students.
And now no one is happy.
Generally it's better to just be smart than to try to prove or weaponize it.
But relatively easy to notice if you take time to watch lectures themselves. Ego driven complexity is different then inability to explain complexity or it is complicated complexity.
In contrast, what students actually need is to transition from one state of mind to another state of mind.