IS's link doesn't state that Comey leaked any of the four memos that may have contained classified information. He showed them to a friend (presumably for advice) - which would be the "mishandling" to which the article refers. And "mishandling" is at present hypothetical, since an investigation is not yet underway. Also, though the timeline for his sharing his memos is unspecified, I would presume that since Comey had his friend leak the one in order to pressure the Justice Dept. to appoint a special prosecutor, that when he did so he was not head of the FBI!