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Thermal lenses create "dead" air space that moderates the air temperature differences between one face of the lense and the other. For a single pane, as the lense chills from the external side the interior side cools the same and water in the humid vapour between your face and the lense condenses/collects on the lense creating the fog. With a thermal lense the dead air space allows for a more moderate temperature difference between the inside/outside faces of the interior lense...and hopefully with enough ventilation...fogging is avoided. It doesn't change physics...it just delays/suppresses the environmental conditions.
If the gas between the lenses was "dry" and inert (like a window for a house) then regardless of how hot the interior lense was vs. how cold the exterior lense was you would prevent the build up of fogging.
Re. water vapour from eyes/face/skin...that's still being expirated and thermal lenses don't change that...but keeping the lense temperature difference close enough is the key.
In the end it comes down to ventilation. Airflow is everything. If you stand around long enough so you cool down and allow your skin to dry off...then you're likely to fog very little. If you run around like crazy in humid weather and then drop down in the shade and drape a shemagh over your head/lenses to cut off airflow then no amounts of fans/anti-fog/etc...will prevent fogging. Obviously those are ideals to make a point and not practical in game play.
And...some weirdo bastards never seem to sweat enough from their eyes/face to fog regardless of what they're using.
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