None of this has ever been true.
http://arstechnica.com/reviews/01q2/macos-x-final/macos-x-3.html
From 10.0, new users were administrator by default. But administrator on OS X (and other Unix inheritors) isn't Windows-style administrator. Windows admin is "root" in Unix--you can do anything anytime. An OS X administrator *can* do most things--not ANYthing--but gets prompted for their password for many things.
The GUI will not let you save a file over a directory, or overwrite system locations such as /usr. Of course, a given application might ask you for privilege escalation and then do awful things, but... welcome to computing.
The comments you saw were from people who either escalated privileges intentionally and then made an effort to do something dumb, like rm'ing /usr without known what it was; or by Windows people who saw the word "Administrator" and out of ignorance assumed that other OSes were as stupid as Windows and would have the default user be the super-user.