so, relative to pretty much all other messaging services, it might as well not be.
You’re saying “by default not everyone can read your messages, only you, the recipient, telegram themselves and anyone who they might decide to share them with, with neither your consent, nor knowledge”
When compared to “nobody except you and the recipient” that becomes effectively equivalent to “nothing”.
also, not end-to-end ever when it comes to group chats
Almost all services in that list are closed source, so even if they use end-to-end encryption nothing stops the client from sending all your messages to anyone they like after decrypting (in fact some of them already have it as a built-in feature in the form of backups).
that would be very quickly caught by a network sniffer, because it would have to be sent from your own device. Otherwise they’d just be sharing the undecryptable ciphertext you sent to their servers
Just encrypt it before sending it to their servers. How would you tell that apart from any other traffic it sends? (E.g. to check for new messages, to update who of your contacts is online, etc)