Domande di colloquio per Python Developer presso Sifted

I was asked a few initial questions on the phone, went great, and the chat was followed by a written coding exercise for which I was given a few days. I sent back the exercise with my (fully tested and documented) code and got feedback that it wasn't good enough and they would go on with other candidates, which surprised me a lot since my code was clean and perfectly working. I assumed they expected more comments, different styling or maybe I had misunderstood the requirements, so I said I understood and asked for some details on the feedback so that I could improve myself for other applications to other companies, since I couldn't see anything obviously wrong with my code. They said they would check with the engineer who reviewed my code but never got back to me. I chased once 2 weeks later, never got a reply. At that point I strongly suspect the task I had to complete was actually a task that was needed on their end and which they gave as a "coding exercise", for which they probably never even intended to hire someone. So basically I worked for free. I can't prove it but even if I'm wrong, ignoring the candidate by not providing feedback is already an issue.