Police have said the guy who walked her to her car is not a person of interest. I think if they say that, they mean it and they have good reason to think that he is not a person of interest.
I have seen cases where they say that a person is not a suspect until they find evidence. I think they said they haven't ruled anyone out, that would include him correct? Either they for sure know what happened to her, or have something on video such as a car racing out of the structure (etc.), that wasn't his, or something. I'm sure they can identify one of the three cars leaving at that time of the morning.