I don’t know what the area with the bollards is for if not loading and unloading. It’s clearly a slip road from the dual carriageway allowing cars to drive on it and there are no signs anywhere suggesting you cannot enter this area.
It is a footway. There is a dropped footway without an apparent purpose because it's built to pedestrian specs and standards, not vehicle, but doesn't have a corresponding 'mate' at the central reservation.
But this doesn't make where you were a carriageway..just look at the surface treatment, they're flag stones exactly the same as before and after the bollards. Your misunderstanding on this point might be genuine, but I doubt it would cut much ice with TfL.
What I don't understand and I think you should mention in reps is why, if you had just opened your boot and not left the vehicle, the CEO did not just inform you and ask you to move. I want to focus on why they served a postal PCN.
If they had started to prepare a PCN to be given by hand why tell you to move and then serve by post? IMO, if you moved at the CEO's request then they have no grounds on which to serve a postal PCN.