Comparing Spinnaker and Kubernetes in this way is somewhat unfair to both projects. The scale, scope, and magnitude of these technologies are different, but parallels can still be drawn.
Just like Kubernetes, Spinnaker is a technology that is battle-tested, with Netflix using Spinnaker internally for continuous delivery. Like Kubernetes, Spinnaker is backed by some of the biggest names in the industry, which helps breed confidence among users. Most importantly, though, both projects are open source, designed to build a diverse and inclusive ecosystem around them.