His write up explains the Delta T as the temp difference of the 200w heat source and the incoming water temp. The higher the thermal conductivity of the paste, the lower temperature difference between your heat source and cooling medium is needed for a given heat flux. Assuming that contact area, heat source power output, and thickness of the paste are constant, deltaT times the thermal conductivity of the paste remains constant. As thermal conductivity of the paste increases, the deltaT required to move the heat decreases. In the graph, the low deltaT is better.