That's a good question, and I don't know the answer, but will be anxious to find out the answer. However if you defined someone as "not subject to the jurisdiction of the United States" how would you deport them, or prosecute them for various crimes? Possibly it's to cover the children of diplomats. I'll bet it takes a bit of research to find the original intent of that clause.
I don't think it can be that Congress can just declare a class of people "not subject" and deny them citizenship, else some future Congress could have declared all blacks to be "not subject" (or all redheads or people of German ancestry, etc).
Here is an interesting analysis:
Gingrich was right: Fourteenth Amendment does not apply to Aliens
Obviously, in the unlikely event that this law were ever passed, the Supreme Court would be called upon to provide us with the ultimate interpretation.