A surprise attack on a full superpower with extensive MAD capabilities is quite possibly the most idiotic move that could ever be made. Milley's call can be summed up as: "you know I'm not an idiot, remember?"
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.