That's the point of the privacy scheme. It would only be able to learn things common to multiple clients. Private data wouldn't make it through the noise.

That is beside the point. No lawyer will allow any "learning" on private data. Even without legal counsel, companies would be stupid to do it without compensation.

How is it beside the point? Legal might initially advise against cloud storage but if you come back to them with one that's encrypted on the client side that likely changes things.

Compensation could be of the form "we get a cheaper rate from Google" or even "this is the only form in which the service is offered" or perhaps "we aren't big enough to qualify for the fully airgapped offering".

The point is, for this to work, your system has to be connected to the internet. And if that's the case, there's no material difference between hosting it on prem and hosting it on GCP (where Google can promise you that it won't exfiltrate data). That's if you trust Google. And if you don't (and you shouldn't - it's an ad / mass surveillance company first and foremost), your only option for sensitive data is to self host and air gap.