Morning drice
I am puzzled by this guidance.
Assuming that this is not domestic premises then the relevant Approved Document B is at Approved Document B (fire safety) volume 2: Buildings other than dwellings, 2019 edition incorporating 2020 and 2022 amendments (publishing.service.gov.uk)
So, it comments on external fire staircases:
External escape stairs
3.31 Where a storey or part of the building has more than one escape route available, some of the escape routes may be via an external escape stair, provided the following conditions are met.
a. There is at least one internal escape stair from every part of each storey (excluding plant areas).
b. In the case of an ‘assembly and recreation’ (purpose group 5) building, the route is not intended for use by the public.
c. In the case of a ‘residential (institutional)’ (purpose group 2(a)) building, the route serves only office or residential staff accommodation.
3.32 Any external escape stair should meet all of the following conditions (Diagram 3.4).
a. Doors to the stair should be fire resisting (minimum E 30) and be fitted with a self-closing device, except for a single exit door from the building to the top landing of a downward leading external stair, provided it is the only door onto the landing.
b. Fire resisting construction (minimum RE 30) is required for the building envelope within the following zones, measured from the flights and landings of the external stair
CONTINUES (My bold italics and underlining)
I think this is one to put to the project design team.
However, my take on it would be that you are altering the doors and even if the building is Listed or other heritage restrictions apply, at the very least the standard should be at least as strict as before the change.
I think that your comment "Perhaps these are fitted to stop fire from entering the levels of each landing of the external fire escape ??" is reflected in the wording of the Approved Document, with the sole exception being the uppermost floor.