How hard is it to untar, take the header and do whatever you want with it (including putting it on github if you want)?

It's definitely a lower barrier to entry than requiring people to create a github account!

Edit: the point being, that it's not just "in the tarball": it's in a specific header file, and the blog post says which exact header file you want.

That is exactly what mercurial users say about code distributed on github. :)

Please respect the author's right to decide how to distribute his code.