Revolut Senior Software Engineer - Backend interview questions

I've applied for a Senior Backend Developer role in Kraków, Poland. I've got a message from the recruiter, then the phone interview was scheduled. During the interview, recruiter described a company, asked about my experience, expected salary and verified English speaking skills (interview was in Polish because it's my mother tongue and the offer was in Poland). After the interview, I've received a coding task to solve. I needed to write an application in Java or Scala with REST API, which is used for money transfer between bank accounts. The application had to be simple, with high-quality code and its correct behavior had to be verified by tests. Data should be stored in memory and no authorization mechanisms needed to be implemented in this task. I also couldn't use the Spring framework. The final app had to be executable as a single *.jar file. There wasn't a strict deadline, but they said that candidates usually resolve this task within 7 days. I resolved it within 4 days fulfilling all explicit and implicit requirements. They rejected my application because I didn't handle concurrent transactions in my app.