If Obama holds another citizenship other than the United States, then US citizens should know about it.
that link is not regarding the constitution. it is a site for international travel and what it does to your citizenship.
But I can find nothing in the constitution that requires singular citizenship to become president in common language.
http://www.usconstitution.net/choose.html
from your link...
” dual nationals owe allegiance to both the United States and the foreign country. They are required to obey the laws of both countries”
There is nothing on that page that indicates a person who has dual nationality can be the President. The constitution is extremely clear; the wording is not subject to confusion.