[auslander]

There is a contradiction: "Although Facebook, which owns WhatsApp, doesn’t have access to the content of those backed-up messages, Google and Apple do." and

" 'iMessage communications are end-to-end encrypted and Apple has no way to decrypt iMessage data when it is in transit between devices,' the guidelines state."

I would say it is the other way around, FB and Google can access the content of messages, Apple can not.

[SanchoPanda]

They may be referring to messages which were sent as end to end encrypted, but were later backed up in Google drive or Dropbox etc. in plaintext. This is something I have done in the past.