@Hyperhidrosis They're both wrong. The SSN is:

1. a natural primary key, you don't use those as your real primary key, you make a synthetic one
2. there are in fact duplicate SSNs because it is a manual process going back to the earliest 20th century. That's why you don't use them for your real primary key. I used to work on a system that would find duplicate SSNs.

Musk is right that giant systems don't necessarily use SQL but they're still keyed on something.