WhatsApp is technically end-to-end encrypted, just not entirely. Your messages aren't visible to anyone else other than you and the recipient (not even WhatsApp), but metadata isn't encrypted and is still collected and visible by WhatsApp (Meta). That information includes the timestamps, who you sent a message, how long did you communicate with a person and such; content of the message remains invisible to them.
In that post Durov mentions how backups aren't end-to-end encrypted by default. That is true, but it's worth to keep in mind that the backups aren't enabled at all by default as well.
What I found hilarious was that in some other posts he praised Telegram as best choice, when in fact, Telegram doesn't have end-to-end encryption enabled by default. Moreover, Telegram only supports end-to-end encryption in their "secret chats" which you have to start manually each time you message someone. Otherwise, your chat are unencrypted.
In the end, if you want truly encrypted messaging services that was multiple times audited by independent security researchers, use Signal. It's the only one verified to be secure and private. Most people use WhatsApp because they had it ever since it launched when it wasn't part of Meta and it was simple and easy to use. This is ultimately why you'll hardly make anyone switch from it.