The IMU altitude only reports the height above the launch point. When the aircraft landed, its final position was -17'. It didn't burrow a hole in the ground. The ground was lower than the launch point.Where did he break 400ft AGL?
I don't have google earth and the terrain map I have isn't very good so I can't judge his AGL but, relative to the launch point, the max height I can find is 395ft.
Look at 17m 37.9s on the log. The IMU reports altitude of 2 feet. Sure, it could be off by some. But the VPS, which is more accurate, has the altitude at nearly 32 feet.
This is the reason the IMU cannot be relied on, and the reason you need to maintain VLOS. At that point the variance is 30 feet! When the drone was at its highest, 391 or whatever, according to the IMU, there is every possibility it was in fact over 400', since it passed over a point earlier where the variance was 30 feet.