That's correct. And yet ... back then, Google couldn't read your messages; Facebook couldn't read your backup; And hackers or law enforcement would need to both get a copy of the blob from Google and the key from Facebook - not insurmountable, but requiring a lot more work and more likely to raise suspicion.

Whereas ... after that change, Google can read your messages, hackers/law-enforcement only need to talk to Google, and yet -- you yourself can't get that backup without impersonating the WhatsApp app to Google Drive; The local backup is still encrypted with a key that only Facebook knows (and would give you, but only if you impersonate the WhatsApp app when talking to Facebook).

I've looked for the logic and failed. Only reasons I can find is that FB wants to let Google index your messages behind your back (unlikely) or some plausibly deniable legal backdoor.