You are correct that routeing cannot be applied unless a pink reader or out-of-station interchange is present to define the route taken. I can only speculate that the routeing mentioned on brfares.com is likely down to a misunderstanding on the part of the fare setter.
The other thing to understand is that traditional tickets are able to stop/start short, so in the case of a double back the fare needs to reflect the most expensive station. With PAYG you define the journey by your touches so you wouldn’t be able to exit at Sevenoaks without paying the fare to there.