As far as I can tell, the aim with Kubernetes was never to replace Borg at Google -- Google is far too invested in Borg, and it would take a considerable engineering effort to migrate away from it. Rather, developers at Google saw an opportunity to create an open source version based on what they had learned and help the world along in adopting the same engineering principles as Google has long practiced. Not all altruistic notions, of course -- Google benefits from the commoditization of containers indirectly, by undermining competitors such as AWS (where containers are still not well-supported) and making their own cloud the best fit for Kubernetes.
Google does run stuff on Kubernetes, via GKE. As I understand it, new products are encouraged to run on GCP. I don't know how many applications they run, however. Maybe someone from Google can comment.