Maybe it isn't, but it's a pretty good yardstick to hold things up to. If you can actually say that no reasonable person would be offended by something you want to say, then say it. If you're not sure, or maybe can think of a few people who might be offended, then you can make a choice. If you still decide to say it, and then someone comes to you and says they were offended, you'll probably want to apologize. Or not; that's also up to you.
At the end of the day it's about recognizing that the things you say and do can affect people in different ways, and being thoughtful about that.