Proprietary as in, relates only to itself, not compatible with anything other than itself. It's a monolith with no standards. You have to write custom software to make anything work with it. Nothing just works with k8s out of the box, because it provides no loosely coupled standard interface that remains backwards-compatible. It has an API that becomes obsolete every 9 months.

Jenkins and K8s do effectively the same thing. They're both monolithic applications (in K8s' case it's a monolith of microservices, but same difference), both have a manager/worker (formerly master/slave) architecture, both run arbitrary workloads, load secrets, store and retrieve logs from your workload, schedule them to execute on worker nodes that you configure, manage users and permissions, etc, etc. They're functionally extremely similar: distributed centralized systems designed to execute arbitrary tasks. The difference is mostly technical. Ironically, Jenkins is the more flexible of the two, with much more stable interfaces.

People in tech think in terms of cargo-cult imaginary categories, like an "orchestration system" - which isn't a computer science concept. Schedulers are, operating systems are, but "orchestrators" are not. It's a term made up to sell a product, like a configuration management engine (Terraform) or an application workload scheduler (K8s). Very different things that people use the same word ("orchestrator") for because it sounds cooler, but doesn't mean anything.