I would interpret such a comment as them saying that it's the 'in' thing or 'trendy' thing to be doing right now.
implying that they believe you're going through a phase and will be drinking again soon. If they knew you as a heavy drinker that will possibly put doubt in some of those you drank with as to how serious you are or you ability to be sober
it might not be personal at all, its possible the person cant comprehend not drinking themselves so don't understand people that abstain.
I wouldn't acknowledge it. I would let my conviction in not drinking talk for me.
eventually I imagine that person may come to ask questions or even advice once they're doubt has been replaced with the respect you are due