I am not sure if this relates, but this past year at Blackhat there was a talk on Messaging Layer Security and they broughtup a concept called TreeKEM, where users share keys in a tree heirachy to reduce the number of shared secrets.
Again, not sure if this is applicable, but the comments made me think of this.
To add to that, the only goal of TreeKEM is to avoid latency/complexity when adding and removing group member frequently/quickly. Which is really not the use-case of most groups (so not sure why they're doing this).
Probably groups with 6 or so members are alright, but the more people your group has, the more joining/parting there is. And the larger the effort is to distribute a new set of keys. So if you want your method to be scalable (and some telegram rooms have tens of thousands of members!), you need strategies like this.
> accept that there is no confidentiality anymore. It's just not realistic to have a "secret" group with that many members
You do have a point, especially when it's a group where people are in their free time. However, if they are present for work they are less likely to leak information. Also, encryption should give a default level of privacy to build on.