In this case... just fast
The piece is coming down and travelling in the same direction as the cars but much more slowly. Wilson comes round that corner at what I loosely termed "oval speed" (very fast) and travels into the piece. You see it strike the upper front of the helmet and bounce back up. It doesn't fall cleanly onto the top of the helmet as it would be unable to. If it
had that would arguably be survivable -
a straight drop onto the helmet is unlikely to be fatal when static and, when moving at that speed, it's going to graze along the top. The impact was more frontal because of the speed of the car, that's clear in the video.