Telegram groups are not E2E.
Chats are encrypted, but the servers hold the encryption keys (I believe).
There are one-to-one chats that are full e2e, but you have to enable it. And it has all sorts of compromises.
Qualifier: this is as dicumented by telegram. Since it's not open source, we can't really verify it
Says who?
"Supposed to be" presumes some authoritative source.