As a general rule, if something is on the HN homepage and you find yourself asking "am I missing something?", the answer is almost by definition "yes" :)
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!)
I really enjoy how people try to dispel their outright attempts at bullying behavior with an emoticon. :)
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.
I think it was an attempt to add a cheeky or comical tone to the response, instead of outright saying "Yes, you're missing something" or the more curt "Yes". But if I helps,
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!)