This question seems not so obvious to me. Or at least, the incentives are reversed from what I assume you're suggesting. If Pinterest is a buyer of Google's ad

This question seems not so obvious to me. Or at least, the incentives are reversed from what I assume you're suggesting. If Pinterest is a buyer of Google's ad services, Google has a strong interest in ensuring that Pinterest doesn't get free organic results. More organic results, less need for ad spend.