Zero gravity does not restrict the evaporation of sweat.
On Earth the evaporation of water is enhanced by air movement like wind.
In zero gravity, there is no air circulation caused by thermal differences but the air in the ISS is circulated with ventilators. So the evaporation could not be reduced by zero air movement. Not to forget the body movement during a workout causing a movement of the skin relative to the air.
Sweat is evaporating in a wide range of temperatures as long as the humidity of the air is low. The temperature and the humidity of the air in the ISS is controlled in a comfortable range, so the astronauts bodies may be cooled by sewating if neccessary. To keep humidity comfortable, water vapor must be removed from the air.