The location of the electrodes aren't arbitrary. The region of implantation is likely the hand-motor cortex (which has thickness on the order of mm), the area associated with arm movements (check out somatotopy), so the signals acquired are actually targeted for the demo task. This relationship has been known in neuroscience for almost a century and has been validated in decades of brain-machine interface experiments.

They are also recording from up to 1000 channels, which is probably overkill for mind pong, tbh. But you'll need that many implanted electrodes to study long term electrode biocompatibility.