The key is that there is a lot of additional services and interface points to handle. As the Coinbase article noted, you need extra pieces on top of k8s (storage, service mesh, config/secrets, etc) that need care and feeding. Even if the company moved 100% of their services into k8s there's now more work to be done for the same level of service.
: The control points that k8s exposes are not simple "drop in your provider here" bits of integration. You would likely still have the same core providers (ex: EBS for storage) but there is now more code running to orchestrate them, and more access control to implement and verify.