I don't see why the author needs to add a "what is D3" section to a tutorial. A visitor to a tutorial site for library_x is pretty likely to know what the purpose of library_x is.
I think he's targeting his audience well. I'd imagine most of his users arrive like this...
1) I need a data visualization library.
2) D3 seems to be a good one.
3) Read the D3 docs at the project's homepage.
4) I still need help learning D3.
5) Google for tutorials.
6) End up at this person's tutorial.
"What is D3" would make sense if users did this in reverse (like I'm sure some people clicking on this news item have done), but I don't think that's the normal use case and I don't blame the author for not addressing it.