Completely agree, it is clearly shown on that map, but it is not mentioned anywhere else I can find. I expect it is an error, the map is clearly wrong as the key includes a colour that is not used. Also there are the considerations, you have a Eurostar train where the majority are enroute to London, it leaves London with perhaps 1 carriage of people to head to Universal, does it then return to London for cleaning as it can't sit on the fast line.
The transport plan is cracking for this scheme, I get off a packed train in St P at 8:30am, the train heading north has few passengers, Universal customers can board those easily, there will also be able to board at Gatwick & Luton Airport. When I board my busy train home, the train heading south with visitors is also mainly empty.