Yeah Telegram only has 1:1 opt-in E2EE, that you can't use across your devices, so either you or your buddy quickly gets tired of whipping out their phone when they're sitting at their laptop, and just replies you through Telegram's non-E2EE cloud chats, and that's the backdoor. The user activated it. It's "their fault".
I'm not going to promote Telegram, just wanted to highlight that Whatsapp is not considered trustworthy by a geopolitical enemy of US. I don't think that Telegram is bad, and when your life depends on it, you can click "Secret Chat" button, it's not a big deal.