Not so. According to the Cortwright report, the explosion happened as the result of degraded insulation on one or both stirrer motors, whose windings were immersed in supercritical oxygen. The insulation degradation was the result of overheating during a pre-launch test detanking operation, using a nonstandard procedure, where the tank was heated above the cutout temperature. The nonstandard procedure was used because the normal procedure would not empty the tank, due to the internal structure having been displaced in an incident at the factory, where the whole shelf assembly was dropped, due to a broken sling adapter.
Normal flight power to the heaters was 28 volts, whereas ground operations used 65 volts. The tank thermostats were qualified for 28 volts, and had not been specified for 65 volts. It was subsequently found that, opening at 65 volts, they would weld closed, allowing the temperature to rise above the designed 80+/-10F
It is believed that the temperature during the detanking operation rose sufficiently to degrade the stirrer motor insulation, and arcing occurred when the tank was routinely stirred in flight, igniting the insulation, and leading to overpressure.
The tank was composed of two hemispherical Inconel forgings, electron beam welded together. Working pressure was around 900PSI, test pressure in excess of 2000PSI. Wall thickness was a mere 0.060". The whole was enclosed in an evacuated outer shell.
The stirrer motors were 200/115 volt, 400 hertz, 3-phase, open induction motors.
The tank heater was not blamed.
The full report is available online, at NASA.