He covered it in the interview in a quite reasonable way. E2E encryption, especially if forward secrecy is a requirement is incompatible with many important features: https://youtube.com/watch?v=qjPH9njnaVU&t=9583

You probably don't realize it, but multi-device support, group chats of tens of thousands of users, channels with millions of users and comments of said users. Attachments and history and search. And then a whole infrastructure for running bots and processing payments. And proxies to fight blocking attempts. All of these are either highly problematic or computationally intensive and practically infeasible with E2E on.

Otherwise, someone would have taken the opportunity and reimplemented Telegram on top of homomorphic encryption /s.