You're right that this really isn't a big deal. It's just something Kerry said on the spur of the moment, probably trying to be funny. But if GWB had said it you just KNOW it would've been front page news.
What we say off-the-cuff in unguarded moments is more likely to reflect what is really in our heart, IMHO. I think that we have to consider Kerry as a potentially unhinged, unstable man who harbors a deeper level of ill-will toward the president that we might have imagined. This is the man who was present at a meeting in which political assasinations was discussed. He did not disassociate himself from that group. This says to me that there's a substantial likelihood that this man has not yet grown up and is still a danger.