All major nuclear powers have systems in place to guarantee the survival of at least some of their strategic nuclear forces in the event of a surprise attack, ballistic missile submarines being one of them.

It would be terrible against a full nuclear triad nation. It’s assumed that the US has nuclear weapons in silos, on airplanes, and in submarines at all times and under certain conditions each silo/plane/sub is capable of making the call to attack.

A surprise attack would only prevent MAD if you were to simultaneously disable each individual silo/sub/plane which is practically impossible.

Oh, and it’s entirely possible the US has classified space based retaliation capabilities as they have had a number of highly classified satellite launches.