Tiger Data Senior Software Engineer - Database Internals interview questions

I was given a take-home assignment to implement a Postgres user-defined aggregate to compute the median. I successfully complete the assignment and even completed some extra requirements with all unit-tests passing. I sacrificed almost one full week of my time (and weekend). I have family and kids and this meant no weekend time with them. I also had to take sick days off from my current work to be able to complete this assignment. Notice I knew nothing of implementing postgres extensions prior to this interview and I still completed the task beyond what was asked of me. Afterwards, they booked a follow-up interview to ask me about my take-home solution. During the interview they didn't even mention my solution. They asked me questions about query optimization, I made it clear to them from the getgo am not a database enigineer (and its also clear from my CV). I answered I answered to my best knowledge. I also asked why are you not asking anything about my implementation. They said that because it was working, there was no need to ask questions about it. They rejected me afterwards which I suspect is because I have less db-engineering experience. But why did they even ask me to complete a very time consuming assignment in the first place? Total waste of time.