Rav Ashi concludes that the distinction between bbq grills that were used for non-kosher food which require libun (our mishna), and those used for kodshim which require only hag'ola (boiling), is the status of the item when it was initially absorbed into the grill. Items that have status of issur at the time they are absorbed require libun to kasher, but items such as kodshim which have status of heter when they are absorbed, can be kashered through hag'ola. Even the inital challenge to this approach by Rabba - סוף סוף כל קא פליט איסורא קא פליט, that at the time of kashering it had already become assur and should require libun, doesn't fundamentally undermine the chiddush of Rav Ashi. Meaning, Rabba would agree that if the hag'ola is done to the kodshim prior to becoming assur as we find in R. Nachman's idea כל יום ויום נעשה גיעול לחבירו, he would agree to Rav Ashi. The problem is that the hag'ola is only being done after the absorbed issur becomes נותר. Based on this, if the issur is being kashered prior to becoming assur, everyone would agree that hag'ola can be used to extract the issur even though it absorbed over the fire (in a method that would ordinarily require libun). Based on this, it is clear that if one would need to kasher a grill that was used for meat in order to use it for milk, they can kasher through hag'ola even according to those who disagree with R. Ashi since at the time of kashering it is still he'teira. However, if one had a grill on which he grilled steak and then 24hrs later grilled dairy on the same grill, Rav Ashi would still hold that it can be kashered through hag'ola. The rationale is that since at the time the meat and the dairy became absorbed into the grill, it was התירא בלע it is sufficient to use hag'ola to kasher, even though now it is a combination of meat and milk flavor that is absorbed in the grill. But, if one would have grilled dairy within 24 hrs of grilling meat, the dairy would not qualify as התירא בלע because the taste of the meat will mix with the dairy and become absorbed as a combination of meat and milk - איסורא בלע. In that case all would agree that libun is necessary.