Well, it takes 47 minutes to travel by train from London Stansted to Liverpool Street and 53 minutes from London Southend to Liverpool Street. Does that six minutes disqualify Southend from its IATA recognised title as London Southend? It's much quicker through Arrivals at Southend than it is at Stansted, so the elapsed time from plane to Liverpool Street is less travelling via SEN than via STN.
Of the six airports recognised as London airports by IATA, only one is actually in London. Even Gatwick is 32 minutes from Victoria by train.......