The answer is pretty obvious - because Window title is not a Swing component, it's OS native entity.
So changes don't have to go through Swing Event Queue, but go to XDecoratedPeer.updateWMName directly in case of Unix and to some other class in other OSes.
The more interesting question would be how to avoid that UI blocking, but I don't think that's possible with just Swing, you'll have to implement some lazy loading, or rendering in batches.