Pete, that's correct, but incomplete - only half of the process. First, note the difference between 'heat' and 'temperature'. Heat is energy, and temperature is a measure of energy per unit mass.
So with greater airflow through the muffler shroud, i.e. more air mass needing its temperature raised, more heat is required to raise the temperature of the air. For a given available heat from the muffler surface, greater temperature rise will occur with reduced airflow.
For that reason, the greatest temperature rise is obtained by running the engine hard, pulling carb heat and promptly reducing power to idle while the muffler is still hot. If you have a carb temp gage, give it a try.