Hmm, I suppose that's true, thinking about it, just the poster seems to be in the wrong place (i.e. I can't see a scenario in which you're "entering" the "London Terminal" being a useful place to tell you it wasn't wasn't valid to "arrive" there). I think the poster would be better saying "are not valid at London Waterloo Underground" rather than "this station"
In the past I've done Norwich-Waterloo, by changing at Stratford and taking the Jubilee line as it less hassle than going into Liverpool Street and on the tube, don't remember it being an issue going through the barriers (it was years ago though)