This document instructs you to use nginx in front of Kibana to authenticate the users. This might have been a valid option when you didn't have security enabled but there really is no reason for you to keep nginx. Kibana can handle the authentication itself.
As you can see from the command you entered, this is not a Kibana user, it is an nginx user.
My suggestion is to remove nginx and follow our documentation for configuring security for the Elastic stack