I would think most people who don't do Santa do explain to their kids to be sensitive to others though. I make sure my kids know not to rain on other kids' parade when it comes to beliefs, or Santa.
But at the end of the day, we don't know what the parents in the OP have said (or not said). Sometimes you can do everything right as a parent and your kids will still say the wrong thing or say something thoughtless that you'd be aghast at. They're not robots.
I don't think its a major issue to get worked up over. My children tell me that other kids tell them they're lying all the time. Its not nice, but all sorts of unpleasant things get said on the playground. They're only little, and they're still learning how to deal with these things.