People say that because more often than not they can't tell the difference between UI and OS (if you think GNOME (or KDE) sucks, does that mean Linux as a whole sucks ?). S60 really is out of date and no plastic surgery can remedy that. Symbian^4 will bring significant changes (though even Symbian^3 could probably be made decent if put on more powerful hardware), with Qt doing away with the hideous Symbian API and that's why IMO a dubious choice that they kept the Symbian name, as it is damaged brand-wise.

Also, there are many OS-es left out from that list, like BlackBerry OS (it's interesting how in this whole OS war BB gets ignored), Bada, S40, BREW, LiMo, you name it.