I don't think this is a fair assessment. Bernie has mentioned marching with Dr. King. He did it at one of the dinners in South Carolina a few months ago. His campaign uses it quite frequently, not to mention people online using it as some time of rationale for people of color to support him. I don't think Rep. Lewis was trying to belittle Bernie's actions during the civil rights movement. I think it is fair to point out, though, that Bernie's campaign has a history of overselling his importance on specific issues. For better or worse, he's been a backbencher most of his career. (Part of that is because he's an Independent, part of that is because he's also to the left of most of the country, far left until very recently.)
You're taking this in a really narrow way. I mean, at what point is it fair to critique Bernie's past, then? He, or at the very least his campaign, have made the marching with Dr. King thing a part of this campaign. They don't get to be all shocked and outraged if someone adds more light on it. I don't think Rep. Lewis is being shady or anything. He's just pointing something out that he has first hand knowledge of.
Hillary never pretended the Goldwater thing didn't happen. She talked about it in her book. I think it's a stupid line of attack. People are welcome to make it, of course, but I still think it's a silly one.