I was under the impression the time referenced was the time from when the “National Command Authorities” gave the order to launch until the missiles came out of the silos, not the missle flight times.
She gave 4 minutes, and that isn’t classified.
The actual time is variable with each stage in the process having time limits, but that still causes variances in times. Launch detection might be “quick” but the time to report up the chain of command and get confirmation of launch can consume most of that 30 minute window, and the 14 minute window for an east coast SLBM means there is zero time.