SHIA Station (Soekarno–Hatta International Airport Station) is located between Skytrain station of Terminal 1 and Terminal 2 of Soekarno–Hatta International Airport.
Unlike the airport train at KL and Hong Kong (as two examples), the airport train is not accessible from the terminals.
By the time they get from Kemang to BNI City, and then get the train, they could have got a taxi to the airport in less time.
For visitors though, and especially those staying within range of BNI City, then the airport train will save you time and frustration.