The user database is centralized (from a talk that lead Skype engineer given couple of years ago), so I too wonder how would a traffic routing nodes affect signing in. I am sure non of the client code has their user database credentials... So supernodes can only prevent communication, calls, video from happening.

So there's something else they are hiding. My Mac skype dies on start since yesterday (trying to sign-in), supernodes would have nothing to do with that.