Yes. I don't think ich cysts need substrate , or anything else, to live and release their theronts. Ich theronts (free-swimmers) are extremely small, able to pass through most filter media. I see no reason that an ich cyst can't release its new batch of free-swimmers by just free-floating in the water column. IMO, although everything imaginable has been tried, I still think killing the theronts is the only way to eliminate ich.
Also, if the sponge filter has been seeded, the tank is cycled. at least for ammonia/nitrite elimination. . No anaerobic bacteria for nitrate though----but in a QT who cares.
