Maybe uncommon over all, but you should definitely expect the GNU project to use Guile for scripting needs. In this context, it's the obvious choice.
Furthermore, anything that tricks you into learning Scheme is a good thing. Like eating vegetables.
I recognized it as lisp right away, but I share the complaint about the format. If you want to write a lisp program then do so. If you want to format something for human readability it's probably a good idea to do it differently. This is just a opinion of course, but I suspect it's not an uncommon one.