No, throttle was surely idle. The plane was just flying vertically. In a dive, you have a 1:1 thrust/weight ratio and can go extremely fast. The pilot reported an overspeed warning on the audio (which people should not listen to if they're sensitive).

My gut says this is a sudden icing incident. Folks are pointing out that they appear to have flown straight into the center of a cell at altitude. A developing thunderstorm is MUCH warmer than the surrounding air, because the water condensing out of the rising/cooling mass buffers the temperature (due to heat of fusion). It can even be supersaturated ("higher than 100% humidity") if this happens fast enough. The water vapor is just desperate for a nucleation site to condense on.

So you have a situation where a metal jet body that is maybe 30C colder than the medium suddenly flies through this soup. It can ice over in seconds to minutes, and the plane ends up with bricks for wings and much more weight than it was designed for, and simply can't stay in the air. Then it dives inexorably.