A digital simulation of an Enigma machine is not just abstract relationships. Its program is, but the program is not the simulation; the simulation is the program running on suitable hardware, which provides the causal element. Note that the hardware on which the program runs is very different from an Enigma machine's hardware, and the latter would be unable to run the simulation program. This is what is meant by substrate independence, a concept poorly understood by many philosophers-of-mind.
There are many differences between Enigma-machine simulation and nuclear-reactor simulation, but the one that is relevant here is that the former's inputs and outputs are abstract information, and we have very good reason to think that a brain's biological purpose (in the sense of what the organism needs it to do) is, like the Enigma machine's, abstract information processing in the broad sense.
There are many differences between Enigma-machine simulation and nuclear-reactor simulation, but the one that is relevant here is that the former's inputs and outputs are abstract information, and we have very good reason to think that a brain's biological purpose (in the sense of what the organism needs it to do) is, like the Enigma machine's, abstract information processing in the broad sense.