The media have widely been reporting this as evidence that Hillary was not telling the truth to the FBI about what Powell told her. Unsurprisingly, the media have once again missed the essential point: there are two kinds of advice one colleague may give another about their professional activities. One kind of advice could be given before the activity, in which case the person given the advice could say that she or he was doing this or that because of a colleague's advice. The other could be given after the activity was underway, with the colleague's advice supporting or ratifying the activity.
Here's a non-controversial example. Let's say I'm teaching a class on a particular subject, for the first time, and I decide that rather giving a final exam, I'll assign a final paper instead. A month after the course is underway, I'm having a cup of tea with a colleague, and she tells me she taught the course a few years earlier, and she always assigned a final paper rather than giving a final exam. In this case, I didn't assign the final paper because of what my colleague said. But her advice ratifies what I was already doing, and is therefore relevant.
Powell's complaint that Hillary or her people are to "pin" the use of private emails on him implies that Hillary is trying to blame him for her use of private email, or that she used a private email server because of what he told Hillary. Hence, his point that "The truth is, she was using [the private email server] for a year before I sent her a memo telling her what I did." That may be the case, but it doesn't in the slightest contradict the ratifying advice that Powell gave to the then new Secretary of State, after she was already using the email. And the fact that a former Secretary of State supported what Hillary was doing as Secretary is indeed very worthy of mention, and shows that any notion that what she was doing was wrong is ex post facto, as was the designation of 100 of her emails as "classified," long after they had been sent.