I had the same problem. Turns out that the air scrubber works like unreal said. You need to pump clean water into it and gives back contaminated water. For that it will take CO2 out of the air, and it'll create a vacuum.
I thought it was a bug as well, that it would take the CO2 (gas) and give O2, but it's not. Its just that the instructions were not so clear (to me)