Perhaps they were killed via silencers and then had their throats slit post mortem as a message. Some killers do have a flair for the dramatic.

The logistics of having an assassination of the original assassins is more daunting to explain. Either the original assassins are to be fully trusted or not trusted at all. This course of action suggests a gray area in between that would not make much sense, as it still involves a degree of trust that they won't simply go tell Michael "Don Corleone, XXX approached us to try to kill you."