"Must know tech X" is exactly the mentality that we try to avoid in our company's hiring process. Most people would claim this makes your job descriptions too flexible, but the demand is actually incredibly specific: developers who can jump on a project in a tech they haven't "used" before and succeed 90% of the time. It's probably the case that only 10% of devs can do that comfortably though. Fits the criteria in 'Who' perfectly!