Indeed, the comparative advantage theorem includes the assumption that being left alone is actually an option that both traders have. It does say that, if two agents have literally any ability to each produce things the other want, they can do at least as well by trading as by leaving each other alone. It very much does not say that it is, in fact, absolutely impossible for one of the agents to do even better than that by killing the other one and taking their stuff.
The Coasian barganing theorem comes a little bit closer, but a lack of leverage can still reduce an offer to something like “if you don’t fight back I’ll make your death painless instead of horrible”.
Indeed, the comparative advantage theorem includes the assumption that being left alone is actually an option that both traders have. It does say that, if two agents have literally any ability to each produce things the other want, they can do at least as well by trading as by leaving each other alone. It very much does not say that it is, in fact, absolutely impossible for one of the agents to do even better than that by killing the other one and taking their stuff.
The Coasian barganing theorem comes a little bit closer, but a lack of leverage can still reduce an offer to something like “if you don’t fight back I’ll make your death painless instead of horrible”.