Maybe they are referring to the fact that Telegram rolled their own cryptographic protocol that had a vulnerability that was fixed with a cryptic message [1] and still has some dubious choices [2]. I am not a cryptographer by any measure, so I don't whether there was a reason why they didn't pick an existing protocol like OTR. But, it doesn't make sense to me that they didn't switch to another protocol that is audited several times but released a second version of their protocol with the same criticisms.
[1]: https://words.filippo.io/dispatches/telegram-ecdh/ [2]: https://crypto.stackexchange.com/questions/31418/signal-vs-t...