Borg != Kubernetes k8s is based on Borg, but they are quite different.

Companies at this scale have integrations between all different levels and layers of the 'stack' that make the use of off the shelf software difficult or impossible.

yes I know they are different, but the lessons learnt/dev in both products probably end up influencing each other.

My point was encryption of services is built into K8s/service mesh and wondering how it fares compared to FB's approach.

This would make a great comparison. I'm not certain whether or not K8's mutual auth supports session ticket resumptions and distribution of short lived ticket keys. The ticket rotation design would probably make a great addition to K8. There are a lot of intricate details in design which can make a major difference in not only performance but also whether or not the system wakes you up at night.

Kubernetes does not have a built-in mutual auth solution