Klarna Security Engineer - Applications Security interview questions

First a logic test, then an interview with HR and finally a work assignment. I applied to their Berlin Branch. I passed the logic test and interview with HR. For the assignent, they gave me a dockerized flask project and ask me to find the vulnerabilities in the code and fix them. I found many vulnerabilities in the code, fixed them as well and finally submitted the project. However, it was rejected under the pretext of using too many external libraries. The problem with this excuse was, most of those external libraries were already included in the original project! Plus, it would have been almost impossible to fix them without using those libraries. Another excuse they used for rejection was me allegedly missing one of the critical vulnerabilities, which was false anyway since i did fix that. So I sent a reply to them explaining all that and asking for more information about my rejection. As you can guess, no answers came. This led me to think that they were just using job offers as a means of having free labor, to get their tasks done for free and not intending to hire anyone at all basically. I have also verified it with a friend who has been working in IT in Berlin for over 7 years. He told me that IT companies, especially startups, tend to do that in Berlin.