My grandma had this same sponge situation but with a dish towel that was more hole than fabric. She kept it for wiping counters and it smelled like old milk no matter how many times it got washed. My aunt finally cut it into rags one night and grandma complained for a week that the "good towel" was gone. Some people just form a bond with gross kitchen stuff, I swear.

Yep, been there. Mine was a pothos that I drowned for almost a year because I thought yellow leaves meant it needed more water, not less. I finally stuck my finger in the dirt and it was soaked two inches down, so I moved it to a spot I walk past less and stopped watering on a schedule. Now I just wait until the pot feels light and it has doubled in size since.

The whole "cut the bitterness almost in half" thing is probably just your taste buds messing with you, because a hot water rinse does not remove bitter stuff from coffee. The compounds that make cold brew harsh do not dissolve any less just because you wet the grounds first, so the idea that a splash of hot water "fixes" anything does not hold up. What likely happened is the hot water warmed up your grounds and made the first bit of your brew act more like a hot bloom, which changes flavor but also risks pulling out more of the sour and sharp notes you were trying to dodge. Water temp matters a lot, sure, but hotter water usually means faster extraction of everything, not just the good parts, so you could be trading one problem for another. If you really want to test this, run two jars side by side with the same beans and do everything the same except the rinse, because otherwise you are just guessing. Odds are the smoothness came from the beans or your grind, not the rinse.