The Guardian article was 'PA agencies' to begin with, and then changed to a named reporter.
This is often the case. Most newspapers can't afford court reporters these days, so they rely on the Press Association. Usually the stories are attributed to PA, but maybe there are particular circumstances in which they add a byline.
The PA reporter will have used the same terminology that was used in court, so perhaps is not entirely to blame. Maybe at some point a subeditor will have made a check, thought "fuck this shit" and added the correct detail about the killer being transgender. Maybe.