It is. But it is open source and the encryption is solid. All communication data is end-to-end encrypted. They have been subpoenaed before and all they could provide was when the account was first registered and when it was last used. The signal protocol is well documented and open source. The foundation and LLC behind it are registered in California and are run by reputable people.

Telegram is run by shady people, supposedly out of Dubai, while it is registered in the British Virgin Islands. Its clients are also open source, however the encryption, if enabled, is of the home cooked variety, although it was improved over time. Unfortunately it is not enabled by default, you need to enter a „secure chat“ for that, which only works with single contacts, not with groups. Despite having access to everything else, and working like a social media-messenger-hybrid, telegram is very reluctant to get rid of clearly illegal content.