At the moment the Flyer buses, airport accredited taxis and certain other vehicles are allowed to access the road that passes right outside the front of the passenger terminal. These vehicles have to pass through two physical security barriers, one of which is permanently attended by security staff.
It's true that at present passengers on alighting from the Flyer buses have but a few metres to walk before entering the terminal. The same applies in reverse further along the road outside the arrivals area.
The video link gives an overview of the proposed expanded airport and it is clear that bus passengers will have further to walk between bus and terminal.
I suppose that this is part of the pros and cons of the expansion. No doubt some people will whinge as they do regularly in the local press when they have to walk the full length of the corridor from the western apron to access or leave their aircraft.
The expansion would remove the road entirely from in front of the terminal. This might be considered advantageous from a security point of view.
It's going to be a few years before we see anything approaching the full expansion. Airport management has said many times that the expansion will proceed incrementally as passenger numbers dictate.