Not quite just a battery but still hyperbole. Basically they interfaced the fungus so that when it detects light, the robot reacts. It's "in control" of the robot, the same way that the light sensor that came with my old Lego Mindstorms set "controlled" your robot...if you programmed it to react to those light readings in advance. The fungus has no control over how its perception of light controls the robot.
I read this as saying "fungus bio-interfaced as fancy sensor".