When you like a candidate, extend the offer. Agree?
As someone points out in the comments, there was never a real job here. This was all done to get free work from the candidate. Iโve had this happen to me once. Had a company that wanted me to re-design their network as an interview task. It was obviously a ploy to get high-level work from someone with more skill and experience than anyone on their team.
I debated whether I should come up with some completely unworkable (but looked like it would function) crap and let them try to put it in place, or to just say โfuck no.โ Just because designing a plausible but dysfunctional network would actually take a lot longer than creating something good, I declined and said they could pound sand. They probably did eventually find some poor, desperate sucker to do it for them for free, though.