I think it's more of a loophole than the idea he was "born" on U.S. soil. By all accounts that I remember, the Kents claim him as their biological son, so if they applied for and acquired a social security number for him, got a signed birth certificate, etc. then he probably is technically a U.S. citizen, even though the basis is fraudulent. Sort of like, "It's not a crime if you don't get caught."