You should read the introduction to the paper. Its very readable and addresses this directly.
Basically the idea is that the demon is actually doing work, but the work is processing the information required to know when to open or close the door. Here the information processing is entirely a function of the state of the protein (the ABC transporter), which detects the molecule it transports by binding to it.
It’s actually a really beautiful idea. Living systems are information processors at a molecular level. That information is is entwined together across quadrillions of copies of tens of thousands of different proteins, building up an organism made of trillions of cells, which itself has higher level dedicated information processing nerve and brain cells
