What Cow said.

Y passengers will be moved if they need space in Y for paying passengers. But if Y is 100% full with staff standby, the J seat may just as well go to staff. That's not our call to make, and the airline provided what it wants.

As far as I know, they will not move Y to J to make room for non-rev Y, who could be anything from a ramp agent to an industry CEO on an ID90 fare, and just as entitled as anyone else.