As a Vim plugin, you get a lot of features of Vim for free (e.g. search, familiar bindings). And the code is only 56 lines. A stand-alone program would likely have less features and be much longer.
Not everything has to be useful or even sane. Especially on _Hacker_ News.
Edit: To clarify, I think this shows what hacking is all about. Playful cleverness and curiosity.
Most of the functionality is in the xlrd library, think of this as just a frontend.
It's possible they eventually plan adding write support using xlwt.