I wouldn't wish to pre-empt an official inquiry, and forgive me if I appear to be stating the obvious.
But if the evidence points to an in-flight break-up, and the weather appears - as the report states - suitable for training, then I imagine the only other reasonable conclusion is that the airframe suffered a catastrophic failure.
Only the official investigation will be able to determine whether or not the aircraft was structurally sound.
I do know, however, of one in-flight break-up of a Seneca in 2002 which was attributed to pilot disorientation, and subsequent excessive manoeuvres. I mention this only because the accident about which you are inquiring occurred at night, over what appears to be featureless and unlit terrain, an environment which can test even highly-experienced crews.