I guess the point is that that whilst the FO might well have no doubt (in their mind) about not letting the captain back in....what is running through the mind of the (possibly very new and junior) FA with his/her hand on the door handle?
Does he/she obey the FO's instructions not to open the door because the F/O is the "officer on deck" and "has the helm"....
or does the FA open the door because regardless of what the "your only the co-pilot"
says it's the captain outside and he/she is screaming to be let in?
As has been said before there's more to this than just chucking an FA onto the flight deck and saying "oh look, problem solved, we've introduced a two on the flight deck at all times rule".......