What's the actual quote from Chris Mathews? If, for example, Chris Mathews said that it wouldn't be neutral for Lauer to intervene in the debate by being a third voice bringing additional information, then that would be true. The debate is between the two candidates and it is up to the two candidates to fact check the other's statements and rebut them. Lauer's role is to provide the questions and ensure equal time for the two candidates.
I wouldn't be opposed to each party providing a fact checker for the debate though, that would probably work out pretty well and not slow the pace of the debate too much.