I completely understand why you're so concerned about this situation. The reasons you gave to not respond to what they said is completely correct. Put me in your situation and I will surely be concerned about just what to say and what not to say. But I just wouldn't pay any attention to them. I'm not sure what motivated you to ask that person to post that for you. I think I would of just left the subject alone and just do what I wanted on the vacation. It seems you still want their approval of your casual drinking. It seems to me they have no intention of giving it up. They have strong feelings toward alcohol just as much as you do. Are you going to be near these two people all the time? There are 6 other people. 5 if you dont count the person that asked for you. Do they feel the same way too? You can drink when they're not around. I doubt you show characteristics of being drunk. They would never even know. Sometimes we have to give in to other people's expectations to get some joy. It might seem like it sucks, but if you're the only one that likes to drink, it seems the only way.
They even said, "Remember to drink responsibly." This seems to tell me that they know you are going to drink. And they give me the picture, that maybe its fine, but just don't overdue it. Don't worry about this so much. This should be a fun trip so don't let others ruin it for you. They're nothing but opinions.
Best wishes,
~jamesish~