I agree,I dont think Sponge filters should be cleaned at all. It would kill the bacteria that is built up in there. Maybe squeeze it in the aquarium water but not actually cleaning it.
I have been using sponge filters from the very beginning of entering this hobby (around two years ago). I have replaced two Hydro Zero sponges due to wear out, but I have NEVER cleaned a single one. I have dozens of IV's & V's that have been running for nearly two years without a cleaning or even in tank rinse!