OK, time for a hypothetical.
I am not claiming this is true, but just to make a point.
Let's just say that a neighbor had a camera pointed from their home to monitor the street, and it happened to capture BC at 2AM pulling the car into the garage and loading a large, heavy bundle, shaped like a person into the trunk. CPD finds this information and while it is not absolute proof, it does show something that is not explained by BC on the morning his wife is missing and later turns up dead. CPD has the video of the loading operation, but later someone says she saw NC running, when the same video shows that NC never left the house that morning. NOW would you still say they were remiss not to interview the person?
I don't know this person, but it could be someone who calls frequently about things, and is known to the CPD. She might have reported other things that were later proven to be untrue, or CPD might have known something at this point that caused them to understand that NC had never left to run that morning, like say, having the same shoe from different pairs of running shoes left behind.
Eyewitness testimony is not the hard concrete thing that many think it is. LE frequently find that if they have 5 eyewitnesses to the same event, they get 5 different stories about what happened.
I am NOT saying the case was perfect, but I also don't think they were a bunch of keystone kops either, especially on the basis of someone thinking they saw her running in an area known for runners.