The author originally wrote a paper provin that printf is Turing Complete [0].
Here is program itself and an actual, excellent write up [1].
@dang, you might consider switching the link to the GitHub. I don't think the Hackaday article actually adds anything new.
[0]: https://www.usenix.org/system/files/conference/usenixsecurit...
[1]: https://github.com/carlini/printf-tac-toe