Well, what can I add that others haven't already said?
It is real. That is the first ever web browser/editor. It was designed to read and create a modified version of SGML documents that included linked text within a document... this is where HTML started. And the name World Wide Web came directly from the name of the first web browser.
As for the authenticity of the image... the only thing wrong with it is that it is in color. But there is a gray scale image on his page too (he states that the screen shot was taken later on a color NeXT system, his original NeXTcube was not a color system).
As for the curser... yes, the Mac OS X curser is the same as the NEXTSTEP curser. In fact, the spinning disk curser in early versions of Mac OS X is identical to the NEXTSTEP spinning disk. That is because Mac OS X is a direct descendant of NEXTSTEP.
Attached is two shots of NEXTSTEP 3.3 taken a couple minutes ago. They show examples of the NEXTSTEP cursers.