Right. I'd noticed that the first image, of the harmonic drive, was wrong. The flexible ring gear can't have a disc closely attached; that would kill the flexibility that makes the whole thing go. In some designs, the flexible ring gear is just a ring. The inside of the ring has to be smooth. The elliptical plug is way too small; it has to fit the ring gear.
Harmonic drives are confusing enough. It's amazing that they back-drive at all.
Anything wrong in the text? I'm out of date in this area, and I was reading this as a nice catch-up. What else did they get wrong?