You're calculating it with speed of sound at sea level. At 80,000 feet the 3,530 km/h speed record is Mach 3.289 according to wolframalpha, still bellow the 3.4

You're calculating it with speed of sound at sea level. At 80,000 feet the 3,530 km/h speed record is Mach 3.289 according to wolframalpha, still bellow the 3.45 claimed in the article. Although it is understandable that this mission wouldn't count for a speed record.