but no-one at the meal understood why Jesus said this to him.
This indicates that John didn’t catch the significance of the dipping of the sop at this time, which also shows why John didn’t denounce Judas to the others. As noted earlier, all of the disciples are saying, “Lord, is it I?,” which indicates that they thought the act of betrayal was an involuntary one sometime in the future, and not a deliberate act by a traitor. They still couldn’t understand that Jesus was going to the cross to deliberately die for all of mankind, so how could they possibly think that He could tell His betrayer to hurry up and do it more quickly?