> Please provide more details about what this step #2 actually means.
If you mean that you did "GRANT superuserrole TO nonsuperuser", this does not make "nonsuperuser" into a superuser; it merely allows "nonsuperuser" to use whatever ordinary privileges might've been granted to "superuserrole".
IOW, the privileges on the "CREATE ROLE" page are not inheritable - inheritance only applies to privileges that are GRANT'ed