This isn't necessarily that surprising, given that the first formal explanation of a wormhole-like structure was the Einstein-Rosen bridge, which formed with a black hole that was connected to a distant white hole which expelled the matter pulled into the black hole. Such models were found to be unstable, however, unless there is introduced some other element, such as negative energy or matter, as a stabilizing force within the hole itself.
Many of the factors that have been normally used to classify an observed astronomical object as a black hole, according to the new study, would actually manifest essentially the same for either of the two objects. Even physicists who believe the current objects are black holes seem to indicate that, with current technology, it's a practical impossibility to tell between them for certain.
About the only way to know for sure is to send a probe into one, but due to the spatial distortions of both phenomena receiving transmissions from such a probe would be problematic ... meaning that, for now, the truth will have to remain ambiguous.