Telegram crypto was redone, and is now using standard primitives. You can verify it at high level at https://core.telegram.org/mtproto or low level if you'd like to check sourcecode in their github. BTW, telegram has reproducible builds since last year.
I'm tired of people repeating 7 years old meme without verification.
At best, the Telegram developers are well meaning but have demonstrated in previous versions of MTProto that they lack a background in cryptography or a desire to consult experts. And their public face—posts like this one—seem often to be hyperbolic attacks on competitors, which is not a great look.
I’m not a cryptographer, so I’m not going to review the current MTProto. I hope it’s awesome and bug-free. But some skepticism seems warranted.
Also, when you use the secret chat feature does that now support multiple clients or does it still establish a secret chat with a random recipient client over which neither initiator nor receiver can control?