They are different things.
App Engine versatile is concentrated on application development - i.e.(you have an application and you want to be deployed and managed by Google).
On the other hand, Kubernetes is more about having your own infrastructure.
Obviously, you'll be able to conjointly deploy applications in Kubernetes however, as it is your "own" infrastructure, you're the one to directly manage how both the infrastructure and the application can behave(create services, create scalability policies, RBAC, security policies...).
In this sense, Kubernetes is more flexible in what you'll be able to achieve.
However, as a developer, you may not be interested in the infrastructure at all, only that your application works and scales.
For this type of profile, App Engine flexible is more appropriate.
If on the other side, you want to manage a complete Container infrastructure, then Kubernetes is for you.