It's the profile matched with the speed and rate of descent that leaves many questions. An emergency descent at around 10 degrees nose down will reach VMO/VNE at idle power relatively quickly without some form of drag device, you will then have to raise the attitude to prevent busting structural speeds. That will generally net you in the region of 3000fpm. To dive vertically, but keep the speed at most 180kts, and decreasing despite the attitude, well that's just odd. Granted there will be some errors in the ADSB plots based on what is received, but even then the descent had to be very steep.