There were and maybe still are speakers that have a coated surround that is usually made out of cloth. This was done a lot in the '60s to '70s and maybe before or after that time period. If you see some old woofers from around then, that has a liquid looking coating on the surround, it usually sags to the bottom. Those are probably what they are referring to. The owners of those usually do rotate them them a half a turn to put the heavy part at the top, and eventually the liquid will sag down again. It's sort of a super thick honey looking compound. This is done on some accordion surround edges of all paper/wood pulp cones to help keep them flexible and to help prevent cracking or tearing from all the movement they do.
