Without context it's hard to say why they had this file (which is really three documents — "The Atom and Eve Story," clippings from TIME magazine, and some kind of transmittal slip with a list of stuff on it), but the only bit that appears to have been considered sensitive is on the first two pages, where it says, "For Art L. From [DELETED]," and then at the bottom of page 2 is another annotation that was deleted. Both are labeled STAT in what is usually the position for a FOIA exemption; I am not familiar with that particular abbreviation but it is common in CIA files. If I were to guess, it is what is usually labeled as (b)(3):

(b)(3) Applies to the Director's statutory obligations to protect from disclosure intelligence sources and methods, as well as the organization, functions, names, official titles, salaries, or numbers of personnel employed by the Agency, in accord with the National Security Act of 1947 and the CIA Act of 1949, respectively.

My guess would be that the blacked out material indicates the name of a source or member of the CIA. Such things are routinely considered classified. Or it might indicate where they got it from in some other way, (e.g. "For Art L., From Your Pal Saddam," and page 2 might be, "I'm so glad we get to share this conspiracy theory stuff in our secret meetings!" — this very silly example is just meant to illustrate that it might be something they got from somewhere but don't want to reveal where, because it could compromise some kind of source).

Note that just because something is in the CIA's FOIA website does not mean it carried a formal classification marking; it just means it was in its files. The CIA (nor any other part of the US government except the Department of Energy in certain circumstances) cannot actually classify privately-generated information though it can, again, protect sources, agent's names, etc.