LTNman
You are correct, I believe, (how much longer can I keep this up?) in assuming that the 500 employees relates to operation of the airport and the airline activities. I don't think it includes the MRO companies etc. operating on the airport although I stand to be corrected if anyone knows for sure. The ATC Lasham, Air Livery, Inflite and IPECO operations are substantial and must amount to well over 500 employees themselves.
I don't seem able to dig it out at the moment but I produced a paper for a local politician a few years back estimating the number of employees that SEN would be likely to generate at 2 mmpa, including MRO activity etc. and I think the answer came out at between 1,750 and 2,000. Maybe someone has comparable figures for other airports to see if 5.4 ppd per employee is as daft as it might at first appear.
Mind you if an airport wants to offer "cruise ship" levels of service it obviously needs cruise ship staffing levels!