Well again that is not the NMB's understanding. In two different mediations when Delta wanted to continue talks without the mediator they had to request permission and outline the disussions. This makes sense because in order to be effective the mediator needs to understand exactly where each party stands and their table positions. Mediators generally only allowed unsupervised talks when their schedules would create excessive delays.
This is also why their ability to ice negotiations is such a effective tool.