A Bioweapon is a biological system, a form of life or a virus, used for military purposes. The definition covers all forms of uses, even such no directly harming or killing people. E.g. agricultural pests or diseases intended to reduce the fighting power of the enemies soldiers.
In the Expanse the term is used for both the unmodified protomolecule and subsequently "protomolecule soldiers" created by a secret research program.