Zuck is on a major PR campaign drive, I would not trust a word he says.
He also said that he doesn't visit Russia anymore, yet a recent FSB leak indicates that he was frequenting there. And before that he heavily marketed Telegram as ad-free forever. And before that there were quite weird populist PR tactics when professional cryptographers pointed out Telegram's crypto is a mess.
YMMV, but I wouldn't trust a single word from this guy.
Telegram's crypto may be weird, as the professional cryptographers you allude to have pointed out; I don't know, not being a cryptography expert. But MTProto 2.0 has been shown to enjoy many nice security properties (including a version of forward secrecy, though one afaik not as good as that enjoyed by Signal): formal proofs available here https://github.com/miculan/telegram-mtproto2-verification/tr... and some peer reviewed papers describing the formal verification effort are linked to there as well. Considering that I think calling Telegram's crypto "a mess" is misleading.