We started building fleet from the ground up with plans to build a higher level tool on top, which would provide the application centric workflows you see in Kubernetes. But like all decisions at CoreOS, we only build stuff we have to build. When Kubernetes came out we quickly started evaluating it for our needs. It turns out it's a great fit, which is why we built Tectonic around it.
Kubernetes adds a lot of value to the container ecosystem and is grounded in the same roots as other CoreOS projects: opensource and highly collaborative. Both CoreOS and Kubernetes want to see our components leveraged outside of our commercial products, but we also feel that we can offer the best experience by combining them into focused offerings for people looking for a complete package.