Earlier quoted context omitted.
It's quite possible the law discussed in that article was broken. The author lists the three cases of relevance: (1) whether Clinton knew she was putting classified information into an unclassified system, (2) did she willfully communicate classified information to anyone not authorized to receive it, and (3) did she remove classified information with the intent to retain such documents or materials at an unauthorize…
And the article lists various reasons why none of those would likely apply to her. The IG's report only shows incompetence on her part, not criminal intent.
The criminal intent part is the FBI's job, and will likely apply, at the very least, to things like Clinton's attempt to delete thousands of messages before handing over the server (messages the FBI was able to obtain anyway due to offsite backups, going by the extrapolations observed and detailed on http://www.thompsontimeline.com/).
Even if she's not found guilty of violating any major law, this kind of obstruction of justice alone is in a class of behavior we impeached her husband over. I imagine the FBI doesn't take kindly to the people they're investigating putting the bureau through hassles like this.