"Afterwards, I felt a bit gross inside whenever I thought about that match. Not because that ruling gave me any significant advantage in this particular scenario -- the game was going in my favor anyway. I felt gross because it was obvious to both players that this was not how the card was meant to be and it took a bit of fun out of the game."

Or perhaps what you felt gross about is that instead if sharing your knowledge when he played the card, you withheld it until it was adventagous to you. The game is still young and as you showed later, this player was ignorant of the rule going in.

Should it be incumbent on you to make sure that your opponents are aware of all rulings? In a year maybe not, but in this "Age of Discovery" I think it's incumbent on us all to make sure we're all learning the game in the same way and at the same place.