Thank you for pointing out that difference. Google drive is only used with Android phones of course, so Google only has potential access to those backups that are made with Android phones. However, the problem applies to iPhones too. It's just not Google in that case but Apple instead who has potential access.

People seem easily to forget one of the most important design flaws in Whatsapp's encryption. Whenever somebody makes a backup of Whatsapp's history that gets synced to that persons Google drive - that backup is unencrypted. While it remains unknown to the public if Google and/or some other entities access those backups, the mere fact that it would be possible in principle, is just... ouch.