"Oversimplified" is one way of putting it. I'd prefer to call it "simply incorrect". The manner in which the ring's rest mass is increased [decreased] is ignore

"Oversimplified" is one way of putting it. I'd prefer to call it "simply incorrect". The manner in which the ring's rest mass is increased [decreased] is ignored[1]. If you don't ignore it, and apply conservation of (relativistic) angular momentum to the box-ring system, you'll see there's no reason to expect a net thrust at all. [1] https://iopscience.iop.org/article/10.1088/1742-6596/1141/1/012131/pdf https://iopscience.iop.org/article/10.1088/1742-6596/1141/1/...