This article incorrectly implies that Telegram is end-to-end encrypted, by putting it in the same line as WhatsApp and Signal. Telegram doesn't even try to be end-to-end-encrypted by default. WhatsApp claims to be end-to-end-encrypted, but it's not open-source, Signal is end-to-end-encrypted.
I sent some people a password reset through them but half of them couldn't get their head around it.
So yeah while it has secret chats, they aren't very useful at all.