fleet definitely does not provide a nice integration with docker, and we're definitely trying to decide how we can provide a better integration story. And yes, systemd is complex, but with that complexity comes great power.
fleet is a cluster-level init system with just enough of a scheduler to handle the fault tolerance of higher-level components (i.e. log aggregation, scheduling, etc). In this case, Kubernetes can be thought of as an application-specific scheduler, as it nontrivially places containers in a cluster. Kubernetes can schedule its containers through fleet's APIs and trust fleet to track the overall state of the cluster.