Look at who they call authentic. Three obvious examples are George Bush, John Edwards and Scott Brown. What they seem to mean is that both have a narrative/image that they present to the public consistently. Kerry is who he is - and that is the complex mix that we all admire and like. In Alexandra's book, she speaks of the need in a campaign to reduce a candidate to a simpler to understand image - and how much was lost in doing that. It was pretty clear that Kerry did not easily do that - he was far more comfortable being himself.
I think that everything we do see of Kerry is an authentic reflection of who he is - whether it was the young, hyper energetic high school or college student, genuinely fascinated with so many things; the young, complex sailor, focused on keeping his men safe; or the ultra serious, moral, angry young veteran telling the truth to the Senate. He is genuine when he gives passionate speeches or talks quietly to people as part of his job representing them. He is also genuine in running a Senate office that helps him prepare for meetings, where he is consistently knowledgeable and helps him produce excellent legislation. It is hard to argue that he is not a talented, natural diplomat. The problem in our twitter brained world is that a diplomat/ wonk/ jock/ guitar playing poet, thoughtful enough to try to see every issue from every point of view is way too hard to fit in the space to define him. (OT, but here's a 1974 link to Kerry speaking to People magazine on the idea of amnesty. It actually sounds like a young version of Kerry and makes it clear why he never easily fit any image the media - at any point - tried to pin on him.
http://www.people.com/people/archive/article/0,,20064476,00.html ) (I really wish I had found this back in 2005.)
Ironically, what the media calls "authentic" is patently almost the opposite of what the word means. Scott Brown did not wear a barn jacket (it was bought for the campaign - though there was a story put out that it was a present from his college aged daughters, which strains credulity) and the truck was bought to transport his younger daughter's horse. Yet those are why he is "everyman". The fact is that I can't even guess what Brown is really like. Very little of the serious man he likely is has ever shown through.
With Bush, the media knew him when his dad was President. They knew he was a pretty useless, often drunk, rather obnoxious guy. NONE of that is part of the image he hid behind for 6 years. I doubt he was as religious as the media said and I doubt he was as "dumb" as the left thought. I suspect the things that drove the latter were intentional and designed to appeal to the anti-intellectual streak in parts of America. (Edwards we all know had no life time commitment to the poor and obviously did not identify with other sons of millworker and likely didn't since high school or at least college.)