Source? My cursory search hasn't found much wrong with the current protocol, though mtproto 1 relied on some rather weak cryptography which luckily got replaced years ago.
Audited and found to be a mess is a complete lie. 0 encrypted telegram messages have ever been cracked. Full stop.
They even took some ideas that people in the crypto had pointed out and updated their mtproto protocol to 2.0 https://core.tlgr.org/mtproto