Is this really something new? If memory serves, Telegram has had it's own crypto since the beginning, and I don't remember anything about it ever being audited by... Well, anybody?
Granted, I don't know how MTProto actually works all that well, but IMO Telegram should've just used Noise or something. Would've saved them a lot of trouble. Although that doesn't really resolve the underlying problem that people think Telegram is secure when it's not (i.e., you have to explicitly enable E2EE and it's off by default), at least last time I checked. I haven't used telegram in years so my knowledge might be out of date though.