In the time period when these systems were installed no one monitored the integrity of j-boxes or conduit connections.
That's total crap. Do you really think the USAF would be so stupid to let their entire defense system fall prey to one Soviet agent with a shovel and a pair of wire cutters? Do you really think you are smarter than all the engineers from all the defense and communication companies who installed these silos as part of the national defense system? Or one crazy guy to short two wires and nuke Minsk?
All the cables running out of the control center towards the silos are protected by a pneumatic jacket. If the pressure changes, they know the line has been messed with and an armored security truck comes rushing out. Ask any farmer in North Dakota who put his backhoe into one. The telephone company has dealt with this sort of thing for decades on analog cable without the benefit of encryption.