I highly doubt it's true. Imagine if a new user just got on, but then say they accidentally spammed (it can happen..) or their sibling got on and thought "heheheh time to prank him", then the user gets banned, learns not to keep their stuff open in front of other people, and soon gets 6000 points with awesome answering skills.
Why would they not be allowed to get expert? Mike even told me that "if he's still around", if I get 6000 points he'll give me expert. I've been banned like 3 times, and I'm not even allowed on the DB server.
So yeah, I'm pretty sure that all users with 6000 points can become expert, no matter their ban history.