OSlash Backend Engineer interview questions

They gave a home assignment which they expect to be completed within 8 hours. Assignment is a full fledged backend app with DB. The app is expected to manage few complex APIs (not simple CRUD). They also expect that app to be hosted somewhere and they say they'll check multiple aspects of the design , implementation and code quality. Also they say frontend and security implementation would be nice to have. All this in just 8 hours. This process seems very ambiguous. I am not sure how they judge without communicating with developer about how they built it or what was the thought process. Their smart engineers seem to get the developers thought process just by looking at the deployed APIs. Very immature way of judging. Atleast talk to developer for 5 mins. No proper feedback is provided. Totally wasted my 8 hours .