I think maybe the Unix machines to which you are reffering to are ones on their backend systems. But from all appartent outward signs and statistics, statistics used in professional reports, show that their frontend machines running webservers, etc.. are running on the windows 2000 platform and microsoft's IIS 5.0 servers.
The link you reffered to quotes facts from 1998. The article isn't about Windows 2000, its about Unix Vs. WinNT 4.0.
I quote from the netcraft site, sorry this doesn't have a date but it's reletively new (2000):
"Hotmail Windows 2000 migration completes without incident.
The migration of the
http://www.hotmail.com front end from FreeBSD to Windows 2000 seems to be complete with all recent requests from the site served from Windows 2000 machines and no evidence of any FreeBSD/Apache machines remaining in the load balancing pool. Microsoft will be pleased with this as the migration was completed in less than a month, without any reports of service disruption, and the site has previously been a beacon for open source evangelism."
Sorry if I came off/are coming off like I'm a jerk, it happens sometimes. Please correct me if I am wrong but I need current/concrete facts.