That is not what the website says...it says that vacuuming is most effective when the speaker is left unplugged for 6-12 hours. I myself don't take that as emphatically implying that there isn't other ways of doing it.
The procedure of shorting the panel power supply is described in all ML panel service literature. It is apparently safe for the speakers, the only real risk, from an engineers point of view, is to the person who is doing the shorting, who could have a shocking experience if done carelessly. Playing the speaker without external power to run down the charge is a perfectly sound engineering approach and there is nothing that can potentially damage the panels. The total amount of electricity involved is very very small...about 5 watts...just the voltage is very very high. The actual time for the charge to dissipate from the panels once the power is turned off is highly dependent on temp and humidity.