Since I read the original article, I'd say they understood the NDA, but they didn't agree with it and took this opportunity to get some clicks.
According to the article, they only sign NDAs that meet the following reqirements:
it must be subject to a specific product or information
it must be limited to a specific date
it must not contain terms that are against journalistic principles
Heise stated that this particular NDA does not meet these requirements, that's all the fuss about.