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I mean the document says the data is accurate "as of November/2020", and the slide was prepared 7-Jan-2021

WhatsApp has its own backups in addition to regular full-phone iCloud backups.

If WhatsApp does not encrypt its content at rest locally, then “iCloud backups” still contain everything unencrypted too.

Reading the document answers this for you: It is a declassified government document originally produced by the FBI and was prepared on Jan 2nd, 2021.