There is a screw that is only accessible while the rack is open. You tighten that screw which forces the plastic cover closed so Yes the rack would be locked to the crossbar.
However, the downside with the cable mount is that I imagine it would be fairly easy for someone to come and cut the cable.
However, if you have aerobars and use the T-Channel mount then I think would be nearly impossible to steal the rack(You can always steal both the rack AND the crossbar together though),