I would naively assume that with the huge number of POPs that Google has and the desire to provide a low latency service, Google would only need to have the que

I would naively assume that with the huge number of POPs that Google has and the desire to provide a low latency service, Google would only need to have the queries that originate from their resolvers have the right local addresses. What might be a problem is that for those addresses there is no geolocation information. Which Google could solve by supplying that to maxmind, etc. Hence the need for the privacy violating subnet option.