The absence of evidence is not the evidence of absence.

Truisms aside, in many cases the absence of a thing which one would expect to see is in fact a support for alternate hypotheses.

ie. "Given a world X is true, I would expect to see Y. Since I don't see Y, that's at least weak evidence that X may not be true."

There are of course plenty of other reasons why you wouldn't see Y, but it's at least some kind of signal.

But it doesn't necessarily have to be considered 'public data' for Google to be able to exploit it. Google uses opt-in by default for privacy settings and has a long track record of being misleading when it comes to privacy. I'm sure there's plenty of people out there who have unknowingly opted in to 'volunteering' their Gmail data or will soon be.

On the other two threads, many people considered Gmail to be "public" because it wasn't end-to-end encrypted.