Do we know that this person was taken in and put in a physical line up? I got the impression we were talking about the man being picked out of a photo lineup.
Also, many people here have been questioning this sighting/pointing out things that question it's validity, so I would think that LE might have some of the same thoughts/would want more than an ID in a photo lineup before arresting someone. They could be working to make sure their case is as solid as possible before moving forward, particularly if they do not believe that Lisa is out there somewhere alive. Or it could be as you proposed that this sighting has nothing to do with Lisa.
I'm not sure about any of this, and I don't know that I even believe that what I typed above is the case, but it's a possibility.