> Now, Telegram doesn't use End-to-End encryption by default at all, does it?

Underrated fact.

Also, no one knows who exactly operates Telegram and IIRC they don't even have an office. But we know Russian authorities have intense interest in it so it's hard to imagine FSB wouldn't figure out who it is and knock on their (or their relatives still in Russia) door. We know that Russian authorities previously banned Telegram, demanded encryption keys and then a bit later unbanned it saying "Pavel Durov was prepared to cooperate in combating terrorism and extremism on the platform".