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.
Honestly, the issue was not about their crypto at all, but about the attitude and how they reacted. It's literally as if someone says "dude, I know a thing about crypto and you might've made a mistake there" and Pavel immediately goes into offensive defense, preaching how they have the best ACM champion PhDs and shifting the burden of proof, basically a canonical Putin/Trump-style of evading an argument.
That's what makes me wary of this guy, not his product.