As far as I can tell, the article to which you refer says nothing of the sort. It says that the email contains information which was classified after the fact. The article is based on the Inspector General's letter (http://i2.cdn.turner.com/cnn/2016/images/01/20/2016.01.14.-....), which says the same thing. Do you have evidence that the information was classified at the time it was mailed to her server? Her campaign says it was not.
This matters because a lot of publicly available information--stuff we all know, stuff you can read in the paper today, or can be easily found on Wikipedia--is nevertheless considered classified information. There is no possible way that a single person can keep it all straight in their head, which is why State staff (and others) rely on the markings on documents (digital or otherwise) to know how to treat them.
According to CNN:
> Two government agencies flagged emails on Clinton's server as containing classified information, the inspector general said, including some on "special access programs," which are a subset of the highest "Top Secret" level of classification, but are under subject to more stringent control rules than even other Top Secret information."
Ref: http://www.cnn.com/2016/01/19/politics/hillary-clinton-email...