From everything I gathered it seems about right. That calculator you showed is measuring airspeed at standard no wind conditions, sure the aircraft is going to exceed Mach 1. Here's something else to look at - the U-2 flies at 60,000 feet - it had about a 10 knot window where it could start exceeding the speed of sound or stall - all at about 400 mph indicated airspeed. Certainly the U-2 is not a supersonic aircraft.