Look at the rate of descent. He had the plane on its back for a while, losing altitude at around 1000 FPS. May have been above Mach 1. Auto-GCAS took over and snap rolled the plane around to wings-level, then pulled up. This is the most aggressive autopilot on a manned aircraft ever.
Yet it won't take over until it really, really has to. Pilots can fly close to terrain.
It's interesting that the GCAS took over based on data other than the radar altimeter. There's no radar altimeter data when the plane is on its back, because the downward-pointing radar is looking in the wrong direction. Note that the GCAS arrows are moving in before the radar altimeter data reappears.
I would assume that it keeps the last known offset between the radar altimeter and the pressure altimeter. It may have also pulled up based on the pressure altimeter, as obviously the plane won't be flying below sea level regardless. But the latter is less clear. It appears to pull up before the "sea level" altitude gets too low, but the minimal reading may look less severe because the pilot increased the pullout to 9G.
It might also mix in data from GPS and topo maps. It's also possible they have some internal dead reckoning system to perform holdover while the craft is doing aggressive maneuvers. There are a lot of ways to skin that cat.
Yet it won't take over until it really, really has to. Pilots can fly close to terrain.
It's interesting that the GCAS took over based on data other than the radar altimeter. There's no radar altimeter data when the plane is on its back, because the downward-pointing radar is looking in the wrong direction. Note that the GCAS arrows are moving in before the radar altimeter data reappears.