Maybe they had someone else in mind who matches his description and has a criminal record but it wasn't the guy on camera. Maybe the person of interest they had has an alibi so they were back to square one after that with "no person of interest". Just having a photo of someone doesn't mean they have a POI. To have a POI they have to have a name and they may not know this guys name. IDK, just tossing some ideas out. No POI doesn't mean no one else was involved. It means they have no names, no leads.
If you read sheriff's direct quote, he seems to be saying they found the guy and he isn't involved in the case.