The heat extraction from the plug does not care if a water jacket is present.The finned thermal mass takes care of that process of heat transfer just fine in the "little" motors.The 911 Porsche
did not seem to mind the lack of water either.

The body does conduct the heat into the head,just as NGK says it does.
The heat conduction path is through the nose and into the metallic plug body.
The heat range is determined by the length of that path.

Adding "plumbing tape" insulation to that path,just adds another tuning variable.