I was stunned when I first saw it back then. Initially I thought it was just a simple parody, like asking you some questions and comparing it with the answers. But it was the result of serious hacking, it was evaluating the Lisp code in your answers for real. The author apparently implemented an actual Lisp interpreter using the scripting language of the NScripter visual novel engine. Although it's a really minimalist Proof-of-Concept, but it even has a mark-and-sweep garbage collector and an incomplete tail call optimization (supported feature list [1]). Source code is under BSD [2]. Don't know Japanese and can't comment on the game itself though...
For those who don't know, NScripter was a standard visual novel engine used by many games in the 2000s-2010s. It also has a FOSS reimplementation, ONScripter, by anime and manga aficionados who wanted to play games on non-Windows. Its scripting language was mainly designed for the "if-then-else" storylines and never intended to be a general-purpose programming environment. Implementing a Lisp interpreter is something that its designers had never imagined.
[0] https://web.archive.org/web/20120109071225/http://lyrical.bu...
[1] https://web.archive.org/web/20160514130040/http://lyrical.bu...