Did you read the page you linked?

Try clicking on the "About IA at NSA" link and you'll find out what NSA means by "Information Assurance":

NSA's Information Assurance Directorate (IAD) protects and defends National Security Information and Information Systems, in accordance with National Security Directive 42. National Security Systems are defined as systems that handle classified information or information otherwise critical to military or intelligence activities.

Or in other words, what I just said...