WordPress!!! I never thought this... Then it must be heavily worked upon on the coding part. Is there any advantage of using WP for such a big site, compared to a custom made script...?
Its not necessary that the admin area wud be located at www.domainname.com/wp-admin .. many people change it for security reasons...
It's definitely not wordpress. More likely a custom coded CMS! Even the page structure is different and doesn't follow the standard loop-based format that wordpress uses.