Quandoo Java Developer interview questions

I've received a coding challenge available on their gitlab account. A lot of the instructions are left to the interpretation of the developer but ok. I've solved the task, added dozens of tests and submitted the solution. 3 days later they replied that they have filled their vacancy. If they wer ein the final phase of selecting the candidates they could have told me not to waste my time on the assignment. No concrete feedback on my solution (although it would take them only 10 minutes) which is really bad since they expect you to spend several hours on the solution but they don't want to spend a few minutes of their own time to tell you the feedback. Very unprofessional and bad experience.