The implication the Bob is drawing is that the structural material that the reactor is made from is transparent. This conjecture is not supported by the possible detection method of plasma related to a current flow between the electrodes as seen in the appropriate current measurement instruments.
But, in the report of the experiment, they say that the resistance was 1 ohm and the measured voltage was 0.105V. Those don't sound like a description of a plasma measurement. It could be the ionized operating discharge voltage and dynamic resistance at high temperature, but you would think they would say this if that is how they determined that a plasma was present.