An earthenware vessel in which it is cooked shall be broken, but if it is cooked in a copper vessel, it shall be purged and rinsed with water.
“[An earthenware vessel in which it is cooked,] shall be broken: Because the absorption that had been absorbed in the vessel becomes נוֹתָר [literally, “left over.” I.e., the food remains within the vessel’s wall (see next Rashi), and subsequently, when the time limit for eating the sacrifice has expired, the absorption in the vessel wall is “left over.” Since נוֹתָר, “left over,” must be destroyed by burning, the food in the wall of this earthenware vessel must be destroyed by breaking the vessel].
purged and rinsed: to expel its absorption. [This is in the case of a metal vessel.] But an earthenware vessel, Scripture teaches you here [by requiring that it be broken,] that it never rids itself of its defect. - [Pes. 30b]
In the case of the metal vessel, I can understand that the absorbed material is removed by cleaning and nullified by the volume of cleaning materials. But in the case of the earthenware vessel, the absorbed material is still there!