This is complete BS and it belongs in the cow you pulled it out of. SSBNs patrol in designated patrol boxes so there are always missiles in position capable of reaching a certain set of designated targets. That patrol boxes exist is common knowledge. If they weren't operating in a constrained area, the SIOP (or whatever it is now) would have to be continuously recalculated to account for the targets each submarines could hit from its current position. You would have to redevelop your entire nuclear utilization plan every twelve hours, including disseminating the exact location of all of the underway submarines to all of the nuclear planners. With designated patrol boxes, and by assuming that the submarine is at the far side of the box from whatever target you are looking at, you will have a constant targeting assignment ready for the submarine in each patrol box, and there would be an orderly transition of target coverage as each new submarine arrived in the patrol box and took over watch duty from the previous submarine. The only change in targeting is if the planners decide to change where the warheads fall, which is irrelevant to the submarine as long as the missiles can fly far enough. Considering the range of a Trident II missile (at least 6,000 miles with the light MIRV loads), the range penalty for a 500 x 500 mile patrol box is quite minimal, but it gives the submarine an area half the size of the Norwegian Sea to hide in. From a patrol box south of Iceland, a Trident II submarine would be able to make transpolar shots at the entire land area of Russia.
Yes, it starts its patrol period when it leaves port, and yes, there is no real difference to the crew, but it is not able to take over those targeting responsibilities until it is in the patrol box. In a no-warning nuclear war, submarines in transit probably wouldn't engage without receiving specific targeting information tailored not only to the situation but to the submarine's location, so they would essentially be a reserve force supporting the action of the alert force submarine-based SLBMs and land-based ICBMs.