Is it common for editors to add to a writer's text? My naive assumption was that they often cut and re-structure and maybe suggest re-wording, but I'm surprised to see an editor actively add content that wasn't there at all
Editors do participate in revisions quite a lot and for them to suggest additions is probably common. To take a writer's work posthumously, change it so drastically that you've reversed the meaning of paragraphs ("I lost at the track" -> "I won at the track"), and then publish it as that author's work without their involvement, I think that's beyond the pale.
The main point is that the process should be collaborative. Both parties are in agreement that the changes improve the result. And, I, for one, had no desire to do the author's work for them.