I work at Meta and interface with WhatsApp enough to know a couple things. First of all, data is encrypted at rest; that's not even a WhatsApp thing, that's literally how our infra works (it's actually an efficiency thing, since deletions requires only deleting the key). So the "source" of the article saying

"Meta can and does view and store all the text messages, photographs, audio and video recordings" in an unencrypted format.

Is either lying or wrong.

Second of all, the encryption is legit. The only time "Meta employees" and "Contractors" are seeing your message content is when someone reports your message; because the person reporting it is sending a decrypted copy.

It may be true that there is some sort of device-level backdoor on your phone, or possibly that there's a remote switch of some sort to send a second copy of the message in decrypted format for some targets, but I have not heard or seen this.

Fyi I use Signal and not WhatsApp, but in general I don't think this article holds much weight