Clear explanation why accurate air speed indication was essential. According to this, margin of error was only ~ 25 knots.
"As the speed of sound depends on the temperature of the air, which decreases with altitude up to the tropopause at around 17,000 metres, the maximum speed an aircraft can fly at safely (before running into speed-of-sound effects) likewise decreases with altitude. Thus, on a chart of altitude (vertical) versus speed (horizontal), there is a point where the positively sloping plot of the plane’s stall speed crosses the negatively sloping line of its maximum safe speed below the speed of sound (Mach 0.86, in the case of an Airbus 330). The apex where the two lines intersect—where the minimum and maximum safe speeds are the same—is known euphemistically as "coffin corner". At 10,600 metres, a fully loaded Airbus 330 cruises (for reasons of fuel economy) just below this critical point in its flight envelope—with probably no more than 25 knots (46 kph) between stalling (through flying too slow) and breaking up in a shockwave-induced dive (through flying too fast).
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With the plane buffeted violently by the turbulent updrafts, the first thing the pilot would have done would have been to reduce speed to ride out the storm. But with no clear idea of what the actual airspeed was—and with no automatic systems to prevent manual inputs from making the aircraft fly too slowly or too fast when operating so close to its coffin corner—the pilot could have unwittingly pitched the plane into an uncontrollable stall."
Aviation accidents: The Difference Engine: Wild blue coffin corner | The Economist