All I can say is that somewhere that was not necessarily true. I personally have watched people torch race engines that had no water in them at all and the temp gauge never moved off the stop. I have also watched the temp peg until water totally gone from system and then the gauge drops with just air, but even the steam must be gone to do that. Now it may be that newer sensors may not necessarily do that, but somewhere they did, as we pretty much knew in the old days the sensor does not read if not coated in fluid. Mayhaps they were not thermistors, or 'thermal transistors'.
I messed up on part of that also because cylinder head temp gauges have no fluid around them at all, they read from the metal itself.
Hell, the more I think about it I'm confusing myself....................maybe I'm just behind the times.