Maybe it wasn't quite at 90 degrees, but it was "vertical" to some effect. Remember in drone racing, the fpv drones go almost vertical, and the don't fall out of the sky. However, I see your point that the IMU would stop the motors at that angle...
Other puzzling aspects of this story: It's been claimed it was "vertical"... I assume that means it's was pitched forward 90 degrees? If so, it was producing no lift, and would drop like a rock from gravity. As others have noted, a pitch angle...