The articles on this whole controversy describe it as an ARG - an Alternate Reality Game. I don't know exactly how this ARG works, but ARGs in general are like scavenger hunts - players investigate what's causing the changes, which give them hints as to where to look next.
So presumably it was implemented as a browser extension so game players would be able to find the browser extension, which would give them hints about what to do next.
If that is the case (I'm not saying it's not, just that I don't know)... why did the extension even need to exist? Presumably "certain sites" are partner sites participating in the promotion. If they are participating and (I assume) they control their own content, why didn't they just invert those words or whatever else they wanted to do with the content when they served it?
I'm very confused about why this needed to roll out as a browser extension at all.