I think you mean that rational agents cannot be successfully blackmailed by others agents that for which it is common knowledge that the other agents can simulate them accurately and will only use blackmail if they predict it to be successful. All of this of course in the absence of mitigating circumstances (including for example the theoretical likelihood of other agents that reward you for counterfactualy giving into blackmail under these circumstances).
I think you mean that rational agents cannot be successfully blackmailed by others agents that for which it is common knowledge that the other agents can simulate them accurately and will only use blackmail if they predict it to be successful. All of this of course in the absence of mitigating circumstances (including for example the theoretical likelihood of other agents that reward you for counterfactualy giving into blackmail under these circumstances).