- the requirements being weird might very well be modeling how weird things often get in reality. Rules are often not recorded and not repeated, and very often they seem arbitrary from the POV of the developers.
- the interviewers hinted that the direction was not what they were looking for
- a senior dev needs to apply his own common sense to understand what the truly appropriate solution is given the stated priorities (readability and maintainability were stated)
- not to mention the fact that if the interviewers don't understand your code, things won't go smoothly for you either way
The fact that they said, paraphrasing "..pick any esoteric language.." might investigate whether, given technical freedom (EG: to pick your own stack) you will build an incomprehensible codebase that will be hard for the company to deal with in the future
I think the author is indeed very clever and I learned a few things from reading this write up (thanks for sharing!) but I think the interviewers were right not be impressed.
If I were you, I'd try to dumb it down for your colleagues.