The person arrested has not been ruled out as they are still under investigation.
So whoever they arrested has not been able to provide enough evidence to show the could definitely not be involved.
Of course, however it is not that common for police to explicitly rule someone out this early in an investigation either. Unless they have a reason to publicly state that someone had been eliminated from their enquiries, you don't usually see that. I think it's fairly safe to assume from the fact that they released him that they don't suspect him to be a killer.