Why do some people use the line “I prayed for you” as if it is an insult?

Sadly; yes, there are people in this world who believe that having the final or last word in will be like getting back at you. You seemed very riled by this event. Let it slide - you won the debate. Everyone there who heard the Prof give you the thumbs up knows you won. Self-righteousness is not exclusive to atheists but is oftentimes, an attitude displayed by Christians too. Human nature is the scourge of life.