Pavel Durov says Telegram has never disclosed private message content in its 12-year history, even during legal pressure and investigations.
Telegram says it may share limited metadata like IP addresses or phone numbers under valid legal requests, but not message content.
Do you consider Telegram truly private compared to Signal, Session, or Matrix?
No. Telegram is not end to end encrypted, by default. Their encryption is only in certain versions of the app. And finally, it's a custom protocol that has never been seriously audited.
Signal doesn't claim they don't share, they claim they can't share the private messages, which is true because it's open source and we can see how the encryption works, and we also know that signa's encryption is always on.
Edit: well, I guess is private messages refers only to "secret" chats encrypted by telegrams end to end encryption, and assuming that their custom encryption isn't backdoored then yes, I guess this claim is true.
But telegram chats are not "secret" by default. It must be explicitly enabled per chat.