SSO Administrator account it's like windows local administrator
well...you can say so for your own understanding but there's no direct comparison between those two.
SSO is an application service residing either on Windows or Linux, where both those OS have local users/groups. And SSO with the help of VMDir is getting this domain called vSphere.local (name can be customised) which also has users and group accounts. but this is an additional directory service gets deployed during deployment of VMware SSO.
SSO administrator account is default administrator of SSO, will then help you to configure SSO to point to external identity sources like AD or OpenLDAP.