I don't know whether it was a bioweapon. But consider that if the goal of a bioweapon is to disrupt your adversaries as much as possible, which is more effective: 1. having it kill almost everyone (including your own people) and then disappear, or 2. having it kill some people, cause fear and panic, and disrupt economies and societies for years on end? If it is a bioweapon, it's been an extremely effective one.