We don't know if this woman is telling the truth about anything. However, as a citizen of this area, I would expect that by now my law enforcement representatives would have had a documented interview with this woman, unless of course they were swamped with leads, which seems not to be the case since they spent a lot of time asking locals to say whether they saw Nancy out or not, going so far as to stop traffic.
Yet we don't know if they did everything they should have or not. All we can say is that this affidavit looks bad toward them and damages the potential case against Brad. There will certainly be more to be said about it. Perhaps forensic evidence will be so certain that it will be reasonable to believe that this woman was simply mistaken. Perhaps another woman will come forward and say, "No that was me jogging that morning." Perhaps the witness was a kook. Perhaps she saw Nancy. One thing though, if LE didn't follow up on it because they were too busy (something we don't know), then I bet it just slid up on the to-do list.