The amount of hyperextension I can do with my thumb looks more or less like that. Maybe a touch farther, but not much.
If you look at George Benson, he’s not very “Bent-son” at all:
His thumb is much straighter than you might think. Notice his arm is coming in over the bridge, closer to parallel with the strings. So he simply doesn’t need a big bend in the thumb. The posture of his wrist and hand in its relaxed state simply points the pick in the trailing edge direction.
As far as playing lines that move across the strings, I included examples in the lesson to show how this works. They’re in the “alternate picking” section at 15:31.
Note also that middle-finger / three-finger grip also works for reverse dart, and you can do that with leading edge pick attack, like James Hetfield. So trailing edge is not strictly necessary for all versions of this.
But honestly the best place to start is not to worry about this technical stuff with grips and postures. Instead, mimic the motion in the air or by tapping on a table. If you can do that very fast with a feeling of ease, then you’re doing it. This is a good test because it can reduce confusion about whether you’re really doing the motion or not.