Tech interviews in a nutshell. Interviewer wants you to implement a sorting algorithm from scratch, probably expecting some elegant quicksort or merge sort with O(n log n) complexity. Meanwhile, you just use the built-in sort method that every sane developer would use in real life. The interviewer's face says it all – horrified that you'd dare use a practical solution instead of reinventing the wheel to prove you memorized algorithms from 1962.
Pro tip: The built-in sort is optimized by people smarter than both of you. But good luck explaining that during the awkward silence that follows.