As far as the repair is concerned, it seems like the cut only goes through continuous ground or voltage planes. So in a sense, it is lucky no important traces or logic lines are harmed in the process.
As for how the board still works when that area is cut out, probably just a degree of luck that none of the traces that were severed were essential?
Chances are the breakage caused a power rail to short and the “repair” fixed the short while leaving one or two debug points inoperable.
From the bottom up, there's a layer of bread, then a layer of ham, then another layer of bread, then a layer of cheese, then the top piece of bread. The ham and cheese are your conductive layers, one carrying current the other the ground.
When you first make the sandwich, everything is good because the ham and cheese are not touching each other. However, when you cut it in half, the bread, ham and cheese all smoosh together at the cut point and the ham and cheese electrically short out. Now, if you are super careful, you can probably go in with a super sharp knife, cut it again very carefully, and make it so the ham and cheese don't actually ever touch.
Apple (or whoever) purposely put a cut in the sandwich knowing the ham and cheese would touch, making the sandwich inedible. This YouTuber comes along with a really sharp knife and cuts the sandwich again making sure the ham and cheese never touch.
It just so happens when Apple made the cut, they didn't cut anything important, other than "smooshing" the layers (ie: they didn't cut through any traces or capacitors, etc).