Telegram is just the middleman between sender and receiver.
When you write on HN, the receiver is HN. That message is transported via E2E https encryption so it's secure.
But because HN displays all messages publicly you can read them after they were received.
This doesn't change the fact that the transport as such is E2E.
Ideally it would be the human at each end doing the encrypting and decrypting. But humans can't be bothered, so we let some code that we know very little about do it for us. Obviously having that code run on the client device (the one in your hand) is preferable to having it run elsewhere (like some web server), but either way the human (the true end) is delegating the job to an entity that isn't quite at the end, it's ever so slightly toward the center.
Things like PGP help to maximize the endianness, since the human has a better sense that the crypto software is legitimate, and can read the code before executing it, although there's still plenty of points of compromise between that code and the human (compiler, Intel ME, etc.) so unless you're doing crypto with a pencil and paper, you're always putting your trust somewhere that isn't precisely the "end."
This doesn't change the fact that the transport as such is E2E.