No such assumption made. If you have a positive correlation in your data, you're probably not placing enough weight on your interviews in your hiring process (i.e. you knew people weren't as good at interview but you still hired them). If you have a negative correlation you're similarly placing too much emphasis on interviews; you're likely hiring people who interview well over people who'd be better in the job.

You might still hire bad engineers, but the uncorrelated nature of your interview/job performance data won't help you explain that.