Those were real. As real as the nuclear silo installations across Western Europe. By the way, the SDI concept was next to useless against those subs.
As far as I remember, the war concepts of that time were based on not allowing the counterpart to launch the strategic (ballistic) missiles. Those were for the retaliation purpose only if one of the sides were to lose the war. The Soviet military concept was based on the principles of:
1. Sending the special forces to each and every nuclear silo to attack the silo personnel just hours before the war starts and thus prevent the launch of the strategic missiles.
2. Disrupt the command and the communication channels of the counterpart so that to prevent the orders to be issued or delivered. That included one-time mass-assassination of the major political figures and their deputies.
3. The sudden one-time shock attack on the air-carriers by means of the nuclear subs destroying the air fleet and getting the overwhelming dominance in the air.
4. The massive ground assault with the tanks which would be almost invincible given the Soviet dominance in the air.
As a result, the entire Europe would be conquered and the US paralyzed. Then the new leaders of the US would have no choice but to acknowledge the expansion of the Soviet Union in the entire Europe given the Soviet Union would leave the rest of the NATO members alone. As a consequence, the major communist revolutions would spark in Africa, Middle East, and Asia. Having NATO forces without decent command and inability to make the hasty vital decisions, those revolutions would most likely succeed. That would leave the Soviet Union expanded over the entire Eurasia and Africa continents. Those would be 'accepted' as the new soviet republics as additions to the already 15 present. The North and South America would be left alone. For how long?