There isn't more material around the outside all the way over the entire threaded section. As the steerer flexes between the stem/shim and the spacer or top of the headset, what is flexing is threaded.

This is like taking a threadless fork and scoring it all the way around just above the head set. It is incredibly stupid.



You aren't even supposed to clamp a quill stem into the threaded part of the fork - the wedge needs to be below the level of the threads for exactly the same reason - the threaded section can't take clamping forces.


The idiot in the video you posted is headed towards a deadly crash when his steerer snaps just above the headset at the thinnest spot of one of the threads. How well it is clamped above that isn't going to matter because that isn't going to prevent cracking between the stem and headset. It will look just like this aluminum steerer:




And this is what it looks like when you tighten a quill into the threaded section. Not strong enough for clamping: