Government-produced content like this isn't subject to copyright, if I recall correctly.

Looks like you are probably right:

"Usually, a work receives copyright protection as soon as pen hits paper. However, a work created by an NSA employee, or any USG employee, as a part of the employee's official duties is not entitled to copyright protection"[1]

[1]https://www.nsa.gov/research/tnw/tnw193/article4.shtml