...shall be considered as natural born Citizens...
You're ignorant too?
That means that those naturalized under the act would have the same protections under the law as a natural born citizen did, not that they became a natural born citizen through the passage of the law.
Boyd v. Nebraska ex Rel. Thayer 143 U.S. 135 (1892)
Naturalization is the act of adopting a foreigner and clothing him with the privileges of a native citizen, and relator's position is that such adoption has neither been sought nor obtained by respondent under the acts of Congress in that behalf.A VIEW OF THE CONSTITUTION OF THE United States of America. BY WILLIAM RAWLE, LL.D.CHAPTER IX. Of the Enumerated Powers of Congress
The mode by which an alien may become a citizen, has a specific appellation which refers to the same principle. It is descriptive of the operation of law as analogous to birth, and the alien, received into the community by naturalization, enjoys all the benefits which birth has conferred on the other class.
I hope those help you better understand what that phrase means.