In the course of an ongoing counterterrorism investigation, the FBI used a tool authorized under the Patriot Act, called a national-security letter (NSL), to request library records. The DoJ has argued that revealing the name of the library will irreparably harm the investigation. One of the provisions of the NSL is that a recipient can't disclose that they've gotten it. The gag order sensibly protects ongoing investigations.
So what does the ACLU do? They get an affidavit from the librarian decrying the use of the NSL, block out some information, but leave enough info that one could if so inclined identify the librarian through her description of her professional activities on behalf of patron privacy.
It's hard to believe that a librarian so intimately concerned with the issue of patron privacy would not be aware of the provisions of the NSL. It's highy disingenuous for the ACLU to post an affadavit chock full of statements such as, "If not for the gag order, I would tell other librarians about XYZ..."
And it's dangerous to an ongoing counterterrorism investigation to post information that will let a bad guy know he's being watched.