Default admin user has always been a member of the administrator group in the previous version of AEM (at least until 6.1 I remember) but that's not the case for AEM6.4.
Even AEM6.4 documentation mentioned that "System administration account and member of the administrator group, with full access rights."
I have always used admin/admin and have admin priveledges on AEM 6.4
the admin user is the superuser in AEM and yes, it is not part of the administartors group in AEM 6.4. This is by design. Please let us know if you are running into any issues due to this change.
That being said, I will log an internal Doc bug to update the documentation.