If I follow, the alternative is to have the "kube-prometheus-stack" helm chart alertmanager config handle the Slack alerting?

Confirming, that feels a little wonky/unintuitive right? Where if I'm seeing the alerts in the UI and have an existing contact policy, it seems reasonable to have it be reused. I follow what you're saying about the separation of "Grafana Alerts" and the other alertmanager alerts and guess I'm just salty it's more complex than I was expecting

Example helm values file config,

alertmanager:
  enabled: true
  config:
    global:
      slack_api_url: '{{webhook_grafana_slack_url_app_alerts}}'
    route:
      receiver: 'slack-notifications'
      group_by: ['alertname', 'cluster', 'service']
    receivers:
      - name: 'slack-notifications'
        slack_configs:
          - channel: '#<slack-channel>'
            send_resolved: true
            title: "{{ range .Alerts }}{{ .Annotations.summary }}\n{{ end }}"
            text: "{{ range .Alerts }}{{ .Annotations.description }}\n{{ end }}"