The station manager seems very attached to the misleading sign next to the reader at the top of the EL escalators (and possibly others, I don't know). It just stays there and stays there despite many people, including
@MikeWh , suggesting that the signs be taken down or amended to reflect the very narrow circumstances in which passengers should tap.
The one and only time I was ever issued a penalty fare by TfL occurred many years ago due to a me following a very clear instruction on a similar sign at Woolwich Arsenal when I was transferring from National Rail to DLR, then being issued a penalty fare on board the DLR train on the basis that I had tapped out. The sign at Woolwich just said "Oyster PAYG users must tap their card on the reader", which is pretty unambiguous. The PF was overturned on appeal, but only at the second stage.
I believe since then TfL has, at least, mitigated the situation somewhat by setting the reader (at least at Farringdon, and presumably at Woolwich Arsenal too) to "continuation exit", meaning a tap by a card that was already "in the system" can be treated either as an exit or as a continuation of the journey, depending if there is a subsequent tap when the person actually ends their PAYG journey. So TfL and GTR probably think that makes the misleading sign okay.