Who else would know besides Moore-linked sources? I can only think of one answer: Berg's own family.
Exactly. So what is their motive to leak it to the press? Do they want to find out what Moore knew because he's not talking?
However, it could have been someone connected with the filming of it (cameraman, editor, etc) that tipped off Salon.
BTW, why Salon? That's like tipping off the "Penny Saver", "Carrier Pigeon" or "Coupon Bonanza" papers that litter the foyer of your supermarket. In fact, more people READ the "Penny Saver", "Carrier Pigeon" or "Coupon Bonanza" papers that litter the foyer of your supermarket.
Actually, anyone in Disney-Miramax who was involved in the decision-making process about distribution would have seen the film. So the list of possibilities is longer than just the Berg family.