You can get a better idea of where we are in our journey to Kubernetes (both already deployed and being developed) from this presentation:
https://www2.slideshare.net/aspyker/herding-kats-netflixs-jo...
The general idea is we're adopting parts of Kubernetes where we can and working to extend Kubernetes in ways that integrate well with the approach Netflix has taken to Compute. We plan to bring the approaches Netflix takes to Kubernetes to the community over time as makes sense for the broader community. This work on leveraging user namespaces is a good example of something that while unique to Netflix is something we would like to see the community benefit from as well.
This isn't my expertise (the cluster orchestration system), but I can answer to the best of my abilities: Titus, today is a system that sits on top of Kubernetes, and uses Kubernetes components to do its thing, but we've substituted many of the systems with our own. For example, closer to my area of knowledge, we've used our own executor / provider along with the Virtual Kubelet project (https://github.com/virtual-kubelet/virtual-kubelet) instead of Kubelet.
We're exploring where we can leverage the Kubernetes ecosystem, adapt components, or help contribute changes back that others can leverage to enable our use of more COTS components of Kubernetes.
tl;dr: We're swapping out the engines while in flight