To be sure I understand correctly, they refers to Delta, rather than to American Express having a large local cardholder base?
Yes. My bad. Because of their close relationship with Delta Airlines, AMEX is primarily considering locations where Delta has a lot of flights, but does not have SkyClubs. (Seems to fit what they've done, with the exception of LGA, right? I don't fly DL, so I don't know where all of the SkyClubs are located).