What do we know about the !Kung "major gatherings" (up to 200 peoples)?

A pdf at the University of Toronto of a chapter by Lee on The Intensification of Social Life among the !Kung Bushmen speaks of these gatherings, and the number of individuals reaching 200 was not represented as an unusual occurrence.

Among the !Kung Bushmen of the Kalahari Desert, availability of water was the key seasonal variable. During the winter dry season (May-August) all the Bushmen were concentrated at one of several permanent water points in groups of as many as 150 to 200 people. In the spring (September November) the rains created temporary pools of water all over the desert, and the people went out to live at them. During the main rains (December-April) the people were dispersed in groups varying in size from 7 to 50 people, who ranged widely as the major food plants came into season

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So the camps would typically collect at the permanent waterholes located in the region. He does discuss one circumstance which can cause an increase in the size of the gatherings however (all emphasis mine):

The Bushman men's initiation camps or choma were held in the winter every four or five years. The choma brought together the largest aggregations of Bushmen. The reason for this is that at least seven boys of the correct age were needed to make it worthwhile to run the elaborate six-week-long initiation program. and one had to call in a large number of local groups-to get together enough boys between the ages of fifteen and twenty. The families of the boys camped together and provided food for the initiation camp throughout the six-week period. When very large groups of twenty or more boys were initiated together, the numbers in the adjacent camps must have been well over 200.

This indicates that there was a periodic increase in the size of the gatherings coinciding with the choma every 4-5 years, or when enough young men were of age. Lee goes on to reiterate however that the larger number of in the gatherings wasn't stricly related to the choma, but was also helpful for the performance of the trance-curing dance.

Even in the years when choma camps were not formed, winter camps of 100 to 200 were common. The trance-dance curing ceremonies (Lee 1967, 1968a) brought together medicine men from far and wide. The curing medicine was thought to be especially effective when many performers entered a trance at the same dance. Since the big trance dances went on round-the-clock for twelve to thirty-six hours, subsistence had to be organized to provide support for the singers, dancers, and trance performers. This was difficult to do unless there were fifty or more adults in a camp, since to be effective the trance dance had to have at least fifteen to twenty adults participating at any one time.

This doesn't address a some of your auxiliary questions, but the way the information is presented here it does not appear to be uncommon for that large of a group to gather.

From comments considering why these larger gatherings might have been considered special:

Old people described the excitement of these events...

You might focus further research on the choma or trance-dance being enabled at/by these larger gatherings, or the hxaro aspect ellipsised out in your original quote for consideration in why these events were memorable to certain individuals. Some of these particular (celebrations/rituals?) operated best with larger numbers of individuals, since subsistence still had to be a major concern at these larger gatherings (from the earlier Lee quotes):

The families of the boys camped together and provided food for the initiation camp throughout the six-week period.

and

subsistence had to be organized to provide support for the singers, dancers, and trance performers

Concerning further comments, addressing why one writer focuses on one issue more than others, I can't say, (and to be fair my exposure to this subject is limited by what can be readily gleaned from the internet during these covid days.)

The lack of mention in other ethnographies concerning these gatherings might be summed up near the end of the Lee article Eating Christmas in the Kalahari (emphasis as always, mine):

“But why didn’t you tell me this before?” I asked Tomazo with some heat. “Because you never asked me,” said Tomazo, echoing the refrain that has come to haunt every field ethnographer