Read the article. It refers to children born in territories where one parent is a citizen. It would seem odd that just because she was 18, it would make her son a non citizen. Odd.
If he is hiding the certificate, it is probably because it lists him as a Muslim. Among other problems, it would make him an apostate, and sharia-following Muslims would be required to kill him if they get the chance. Would make diplomacy with the Saudis tough.
Of course, he could explain to them in private that he remains a Muslim, but to gain the Presidency, had to resort to a little taqqiyah.