Both: He's not seriously proposing that TeX is a good choice for the task, but he was serious about accomplishing the task, and he managed to turn it into a paper for publication.

In this case "controlling a rover" was the task of the 2008 ICFP programming contest, and he thought it would be "interesting" to write an entry using TeX. While he wasn't the highest scoring entry, he won the judges' prize for his unusual approach.

A more practical explanation of the project, from his blog: http://sdh33b.blogspot.com/2008/07/icfp-contest-2008.html

It's a joke that he took seriously enough to actually program a mars rover in TeX. I am both amused and awed.