Swift Vets "unsubstantiated" acc. to whom?

Here--from the AP, is an op/ed, er, news article about the Swiftboat Veterans for Truth and their largest benefactor, Bob Perry--who has started another "527." The "reporter" asserts:

The group is a so-called 527 that is not subject to conventional campaign finance restrictions and can spend unlimited amounts on election advocacy, similar to the Swift Boat Veterans for Truth. In the 2004 presidential race, the group of Vietnam veterans made unsubstantiated allegations challenging Kerry's record of wartime heroism.

I guess eyewitness testimony from numerous sources isn't enough to substantiate the criticism? And how rich is it that Ben Evans refers to Kerry's military record as "wartime heroism"?

But with such compelling defense of his record/heroics, Kerry may not be just talking trash when he says,

“I’m prepared to kick their ass from one end of America to the other,” he declares. “I am so confident of my abilities to address that and to demolish it and to even turn it into a positive.”

Oookay, Senator.