Because Sec. 1409 only applies to out of US births--if he was born in Hawaii, he wins. He is both a citizen and eligible to serve as President.
Only reason Sec. 1409 is at issue is because of the assumption that he was born in Kenya.
Reason for the fake Birth Certificate is because there either isn't a real Birth Certificate available for a birth in Hawaii or alternatively a certificate of the data on file from August 8, 1961 will show information that casts doubt on the legend of Honolulu birth.