Are you sure people are not confusing this new proposed air-rail link with the Luton-Dunstable Guided Busway?
There has been proposals for the actual busway to be extended up to the airport. This no more than a glorified bus lane, only buses equipped to go along a busway can use it.
Is there room to squeeze a bus lane under the taxiway, even a single lane with traffic lights to control the direction under the taxiway?