Why do runner docs say 'can't be run in a container'?

[larryclaman]

Why does the documentation file https://github.com/actions/runner/blob/main/docs/automate.md insist that the runner cannot be run in a container? It says:

Why can't I use a container?
The runner is installed as a service using systemd and systemctl. Docker does not support systemd for service configuration on a container.

Yet, elsewhere in GitHub documentation, eg About self-hosted runners, it says "Self-hosted runners can be physical, virtual, in a container, on-premises, or in a cloud." and there are lots of examples of runners in containers, eg https://github.blog/2020-08-04-github-actions-self-hosted-runners-on-google-cloud/

Can someone clarify for me what the runner documentation is trying to tell me to avoid?