That does not say 'citizens of the United States’ cannot be President.
It just limits the Presidency to the subset of citizens who are natural-born. Like Barrack Obama. Or Mr. Ark (but not citizens like me, who were naturalized).
The class of citizens includes both natural born citizens and naturalized citizens. Only the former can serve as President.
It most certainly does say that.
“No person except a natural born citizen, or a citizen of the United States, at the time of the adoption of this Constitution, shall be eligible to the office of President;”
This means you cannot be President UNLESS you are a natural born citizen, UNLESS, of course, you were a citizen of the United States at the time of the adoption of the Constitution.
The Framers were ‘citizens of the United States’. They were not natural born citizens of the United States. They had to grandfather themselves in.
The clause, ‘or a citizen of the United States, at the time of the adoption of this Constitution’, expired by the time James Buchannan was running for President.