A dust free environment means a confined space with dust-free air sipply from HEPA Units filtering particles down to um-level and a positive air pressure to make sure that airflow is one-way-out only. It could be as small as a brench or as big as a factory. A normal working space along with a air purifier sitting at the corner or thorough cleaning is definately not a dust free enviroment, far from it.
But in reality, there are some people who open their e stat driver, put it back and nothing happen. So whether dust gets into the driver or not is simply by chance. But if you do not want to throw the dice everytime , a dust free environment is definately needed.
Some headphone makers revise e-stat designs from the typical stax type to make it less prone to dust contaminations so a dust free enviroment might not be necessary, but not for stax drivers.