If this is the first place your head goes, I don't know what to tell you except perhaps that this paranoia exhibited from the security community is often not rational, and frequently resorts to takes-no-prisoners stakes.
Here's an article [1] that goes over the attempts at blocking Telegram after the FSB demanded the encryption keys, was denied and the collateral damage that resulted from Roskomnadzor attempting to enforce that ban.
I ran this by my Russian friend and he confirms TJournal is reputable. However, he also cautioned believing a known propagandist, deputy Matveychev who made these claims. And toward the bottom of the article you sent:
"A source close to the creators of the messenger, however, doubted the deputy’s statement: when asked what Telegram thinks about Matveychev’s statement, he replied: 'Clowns.' This was reported in the online publication 'Durov's Code'."
It's difficult to believe that Durov who was driven from Russia and from his first company for refusing to hand over information on Euromaiden protestors would so jeopardize the trust he's built over the last decade by allowing hardware backdoors.
All this about him refusing to co-operate is just Durov's words. So you choose to trust him for some reason. But that's not how security works. Zero trust security model exists for a reason. Moxie is right, Telegram is not secure.
Here's an article [1] that goes over the attempts at blocking Telegram after the FSB demanded the encryption keys, was denied and the collateral damage that resulted from Roskomnadzor attempting to enforce that ban.
[1] https://www.bloombergquint.com/view/telegram-block-gets-help...