700s are route certified for all kent routes. If they were diverted that way, they would need the ability to stop.
Would they need to stop though? EMT HSTs and Meridians are (obviously) cleared for the Midland Mainline but you don't see stop boards for them other than at places they are regularly booked to call. EMT don't do special stop orders at places like Hendon and Radlett. In the same way you don't see 375 coastal units stopping specially at St John's, although of course if they did they could use the existing stop marks.
I don't see why they would need to put up stop boards at places they were never booked to call. In a dire emergency surely the 8 and 12 car stop marks would more than suffice. 700s are cleared via the Mid Kent too but you don't see RLU and FLU boards along there.
I can only assume they stuck them in because there are aspirations to stop 700s at these particular stations at some point in the future.