Telegram’s End to End Encryption by default is disabled, only available for one-to-one conversation. My ‘guess’ is that most of group conversions are pretty accessible on telegram servers, which might be the case here.
This article repeatedly referred to Telegraph as "encrypted" ("have full encryption on their services", "when you’re running a globally accessible encrypted platform"), which seems misleading, if by default, it is not end-to-end encrypted.
I think the idea is that Telegram servers are encrypted with keys that Telegram itself has access to (just distributed to different countries) - as opposed to e2e encryption.