There are different levels of stuttering. Some people have very minor stuttering that they can easily mask over, perhaps it only comes out when they're very stressed. These people probably identify as having a very minor stuttering problem, and want to share that with you to establish some kind of common ground and to make you feel more normal.  I think you should take it as it was intended.  Personally, I don't really like this view of stuttering. I think if your stutter is so minor as to be completely masked without noticably affecting your speech pattern; it's just so vastly different from my own stutter as to be incomparable. But even so, I wouldn't 'challenge' anyone that shared it with me, I would smile and move on, accepting the friendly intention all the same.