It doesn't seem like the wet bulb model accounts for ratio of surface area and volume.
The instrument they use is the sling psychrometer, a long cylindrical tube [1]. If I had to hazard a guess, the ratio of surface area to volume is much higher for the bulb than it is a human.
Evaporative cooling is a function of surface area--more surface area => more cooling capacity. I'd be curious to see how accurate the model if the surface area to volume ratio were more human-scale.
1. https://www.sciencedirect.com/topics/engineering/sling-psych...