Apparently you also would convict of murder the person who pointed a gun at someone, intending to kill them, then thinking better of it.
The law is not about what could have happened, but what DID happen.
You are correct. I checked the letter of the law for piracy, and it does specifically require robbery, which specifically requires taking possession of property. All they are guilty of is attempted murder. The law should be changed by Congress to reclassify attempted piracy or attempted murder for the purpose of piracy as piracy, but they cannot be convicted under the law as written.
BTW, I think we should have the death penalty for any attempt at a crime for which the death penalty is a penalty. I also think we should have the death penalty for rape, kidnapping, and piracy.