True, but it still makes the attack surface much smaller - employees of neither company could steal your data. Your data is now protected by the intersection of the companies privacy policies rather than the union of them.
It used to be that way. But then, one day, Google announced that WhatsApp backups (a) no longer count towards your quota, and (b) are no longer encrypted.
There are two beneficiaries of this change:
1) Intelligence and law enforcement agencies, which now have direct access to WhatsApp history for everyone who uses cloud backup (99.9% of users, if not 100%), without the need to 0day any specific phones, risk detection, or even have those phones on except occasionally.
2) Google, who can now mine your private conversations, metadata, etc.
(At a tiny storage cost for Google, for which they are likely compensated by the NSA)