Answer
ArgoCD RBAC governs actions within ArgoCD (who can view/sync/delete apps, manage clusters/repos). Kubernetes RBAC governs access to cluster API resources. A user might sync an app in ArgoCD (ArgoCD RBAC) while ArgoCD's service account applies the resources using its Kubernetes permissions.
Real-world example A developer with ArgoCD sync rights triggers deploys, but the actual apply uses ArgoCD's own Kubernetes permissions.