While Mesos always supported a number of container orchestration layers (e.g. Netflix's Titus), you are right in that Marathon used to be the only container orchestrator for DC/OS up until 2017. Mesosphere DC/OS has had a Kubernetes package in GA since March and we've had 6 releases since then to make sure always was newest version of Kubernetes and to add features (Strict Mode).

Disclaimer: I am the PM at Mesosphere.