Yeah these though experiments all seem to depend on some kind of very tiny perfect subatomic process which has no energy input. Flip it around and the violation of conservation of energy should place a minimum requirement on the energy required to run a real Maxwell's Demon, or the mirror in this experiment (and real mirrors aren't infinitely thin mathematical abstractions).
A good way to ask "what does our model mean, exactly" is to imagine the perfect processes with no unnecessary energy losses. Real mirrors might not be infinitely thin mathematical abstractions, but if there isn't an actual, physically defined fundamental limit to how thin a mirror can be, then it would be weird if we could violate fundamental laws of the universe, but for want of such a mirror.
Maxwell's Demon is neat because we can whittle things down and eventually get to needing to account for the physical cost of the bits in the thing's 'brain.' An interesting example of the fact that information is actually a physical quantity.
