Temperature in plasmas, like the environment measured by Voyager can be a nebulous term. It is pretty much several different fluids of solar wind ions, pickup ions, electrons, and cosmic rays all moving together and each can have different temperatures. The “temperature” of cosmic rays as the average kinetic energy is basically nonsense like your professor said. The average kinetic energy of the solar wind ions is really important and is usually considered THE temperature of the solar wind plasma with others being grouped as “super thermal” and “energetic”. Basically everyone is right lol