Factcheckers themselves can be bull****ters. I think a true test of that (besides going through the list of claimed lies) would be if the same factcheckers have looked at other candidates. If they haven't, I think its indicative. This very much seems to be the case with Benen. Cheerleaders have been known to try to pass themselves off as fact checkers.
Benen's bio is that he's a producer for the Rachel Maddow show and has been a guest on several liberal programs. I certainly wouldn't take such claims on face value, especially from someone with that kind of pedigree. Smells of politics.