A guy asked me if I want to go to a sold out art exhibit with him on Friday for a first date..but he mentioned he had bought tickets and his friend bailed on him. Did I respond appropriately?

We were chatting about art (his interest) and things were flowing but I hate that he invited me as a back up. So I responded that it sounds nice (I actually do want to attend) but that I am not a back up/fill in. I’m not sure why he felt the need to mention I was a fill in. Am I overreacting?