A good question 320 Driver, especially as an inadvertantly entered flat spin is about the only thing that could bring the aircraft to where it ended up at the minimal rate of descent required to produce the evidence found to date.
In other words, a flat spin is the simplest answer that fits the evidence as we know it.
I don't doubt that modern aircraft are supposed to be spin resistant and have all sorts of ways to prevent getting to a stall, but we all know strange things happen in the vertical and horizontal movements in CB buildups and it is not difficult to imaging how a storm could take over and induce a high angle of attack, perhaps yaw, the crew lose control and a flat spin result.
I wonder if either of the crew on duty had any spin recovery training at all? It used to be a requirement for a Private Pilot license, but I hear that is no longer so, perhaps because a spin is an out of control manoeuvre and training is all about control. Equally, who would have recognized a flat spin anyway - would the instruments tell you? How would today's AHRS electronic instrument sensors translate the forces of a spin? It used to be that the only dependable clues were the slip indicator and the rate of turn indicator (turn & bank) and airspeed...
I should add that a flat spin produces an extreme rate of yaw as opposed to a "normal" spin which might make one wonder about the Vertical Stabilizer and that in a spin, at least one wing is stalled, and the two wings are operating at very different angles of attack.
I could go on, but would not want to be distracting by throwing too much aerodynamic fact into the frey.