If Onidah was an obsidian outcropping, then these rebels gathered there and refitted their weapons with new (sharper) pieces of stone. If it actually was some type of armory, then they acquired already-made weapons. However, given the lack of a standing army, an actual armory seems like an impossible specialization. A much likelier scenario is that each man made and maintained his own weapons.
However, after reequipping themselves, they moved to the “mount which was called Antipas,” a location that obviously gave them the defensive advantage of higher ground. Almost certainly, they were a small body facing a larger army. Thus, they needed every possible advantage.