It is built by an anarchist person who always refused to give out personal info of people to the point where his company was taken from him, and he ran away to France. He and his brother hired a ton of PhDs and made lots of paid challenges that no one could break their encryption. Their software is open sourced. The encryption Moxie Marlinspike uses was conveniently funded by the very US government agencies interested in breaking encryption.

https://surveillancevalley.com/blog/government-backed-privac...

https://twitter.com/durov/status/872891017418113024?s=21

Or if you want just a track record of the most RECENT bugs and backdoors that were exploited, look two months ago:

https://www.helpnetsecurity.com/2021/01/21/bugs-video-chat-a...

Find me the same for Telegram!

Signal is a US company and „do not roll your own crypto“ is an NSA meme. It translates to „only use crypto we probably know how to compromise“. That doesn’t mean the algorithm itself must be wrong, it could just be that the implementation has subtle bugs.