Btw, the doc page you linked does say
This account is not meant for individual users and does not have permission to log in to Kibana from a browser. Instead, you’ll log in to Kibana as the
elasticsuperuser.
Right after
You’ll configure Kibana to use the built-in
kibana_systemuser and the password that you created earlier. Kibana performs some background tasks that require use of thekibana_systemuser.