Zalando Backend Developer interview questions

First contact was from email and a call with the HR for initial informations and doubts about the process and then the second call that I had was with an engineer for the code test.

The process took 2 months. First I was given a programming test followed by an HR interview, later a live coding round with an engineering lead which went quite well. In the Final round, there were back to back 3 interviews each of 1 hour in which first interview was taken by a Senior QA Lead with 0 technical questions, next one was a system design interview where I was given a problem to design a system related to Payment Gateway by a senior zalando engineer, in the last one I was asked to improve the solutions I submitted in the first round along with few javascript questions like - Difference between es5 and es6 this, why is nodejs asynchronous, what is callback hell. Both technical interviews went quite well overall it was a very good experience, but I was vetoed by the QA Lead who never really asked me a single 'development' level question but all either culture or team-bonding related questions (like what would you do when a fellow developer interrupts you during your group discussions etc.). I can't quite understand why did they assign a testing executive to interview a backend developer.