Earlier quoted context omitted.
This isn't how the launch orders are issued. Submarines do not have a voice channel to the president, or to the military command. They receive coded signals that instruct them to take particular actions. Creating the appropriate coded signal is a fundamental part of the launch control chain. Also if the president is killed, and the vice president and the head of the senate... eventually the question is "who has the c…
The codes used to signal submarines are demonstrably not the “nuclear launch codes”. The military hierarchy is established and chain of command is already established for contingencies like “who gives orders when the top brass are assassinated.”
No - they are not the launch codes, but the submarines need a launch authority. Apart from the ones that listen out for The Archers on BBC Radio 4 and then open a letter in a safe.