I'd have to agree with Alumnus and Super! My mom doesn't remember half of her sorority stuff and I think my dad does to an extent...but either way it's not AS important to them anymore.
You know, it's really funny because my sorority is in my thoughts consistantly every day. What's going to happen once I graduate? I'm sure I'll remain an active alumna, but I'll also be moving onto a new stage in my life so it won't consume all my time. Who knows! I too will probably forget little secrets and their meanings. But as long as I still recognize and appreciate the sisterhood, who cares!?!
As far as the question at hand. I'd blow it off. The only time I question people is when I'm chatting online and they confront me by saying they're a memeber of my sorority. You can't be too sure. But since there is proof that this famous person was an initated member then it shouldn't matter!