If you look at a sprocket -- any sprocket -- that only runs in one direction, as does a freewheel cassette, you come to realize that the chain only runs aginst the backs' of the teeth. That means you;re free to do whatever you want to the front sides, as long as the back cradles the roller so it won't climb up, and the next roller can swing into position without touching the next tooth.

Since the rollers are attached to each other, each travels in an arc with exactly 1/2" (0.500") radius and has to clear the tip as it swings in. The new sprocket meets this condition, even though the front of the teeth were sculpted away.

Now consider a worn sprocket. As the chain wears the pin wear allows the links to move apart (stretch) so the worn pitch may be 0.505". As this worn chain runs on the sprockets the arc that the rollers is now larger, so while it wears the backs of the teeth based on that pitch, it can sweep by a corner based on that 0.505" radius. All well and OK, until you install a 0.500" pitch chain with a smaller swing radius. It needs 0.005" remover from the unworn corner so it can swing into position as it engages.

This is hard to see with the naked eye, but the wear pattern is visible to anyone used to seeing sprockets, and can be confirmed as I describe using a loaded chain and testing whether the rollers touch the corners.

If you're having a problem visualizing what's happening, find a door that opens close to a piece of furniture. Move it so the door has the minimum toom to swing by the piece without touching. Now move the furniture over another fraction of an inch and try to open or close the door. If you moved it only a hair the door can swing by, but it touches the corner as it sweeps past. Any more and it won't swing past at all.

BTW- look ar the new sprocket (back of teeth only) and compare to the teeth visible at 6:00 in the OP's photos and you should be able to see the worn pockets on the teeth, and the beginnings of the offending hook. Hint, compare the worn teeth to those on the neighboring sprockets, and the difference becomes more obvious.