I just skimmed the paper. Right in the end of the discussion, the authors write:
Our model provides a rigorous demonstration that continuous thermal power can be supplied by a Brownian particle at a single temperature while in thermodynamic equilibrium, provided the same amount of power is continuously dissipated in a resistor. Here coupling to the circuit allows electrical work to be carried out on the load resistor without violating the second law of thermodynamics.
I don't find this very clear at all, but it appears to contradict the claim that the setup could be used for energy harvesting.
Our model provides a rigorous demonstration that continuous thermal power can be supplied by a Brownian particle at a single temperature while in thermodynamic equilibrium, provided the same amount of power is continuously dissipated in a resistor. Here coupling to the circuit allows electrical work to be carried out on the load resistor without violating the second law of thermodynamics.
I don't find this very clear at all, but it appears to contradict the claim that the setup could be used for energy harvesting.