I thought a simple interpretation of his statement was simply that in whatever database he's referring to, there's no table in which SSN is the primary key. That is, it's not a primary key at all and multiple people can have the same SSN by design. Maybe I'm missing some context from the whole thread, but from just the screenshot I can't see why this implies Elon doesn't know SQL.

It is as you say, it's almost impossible that he doesn't know how SQL works...