Am I missing something? This is just a toy implementation of a function prototype, that only includes integers and strings?
It's just a cool use of some of typescript's more advanced features that many developers probably don't use on a day-to-day basis (likely for good reason, as other comments have pointed out!)
Meanwhile, the HN homepage is not some carefully guarded display of exceptional merit, and no serious "hacker" would take the things posted here to be above reproach.
Yes, you're missing something.