True, there's an ambiguity in "visible light" between "EM radiation within the visible frequency range" and "EM radiation within the visible frequency range which is of sufficient intensity that we can detect it with our eyes"

But this is independent of the misconception that the radiation observed in this experiment is thermal. Thermal radiation in the visible range at this temperature is much lower in intensity than the biological radiation observed here, but both kinds of radiation are well below the intensity that we can see with our eyes.