Technically, it should be no harder than the "delete" button, which has been the de facto "edit" button up until now. The article itself talks about how defining the feature from a product perspective was really the hard part, and I'd say it makes sense to take that at face value. You really do want to be able to see when a text has been edited, there do need to be some limits, etc. and defining those things is not trivial by any means.