If you're wondering why 1004, a seemingly random number, is so close to the top of the list -- something that the author does not investigate in any detail -- my guess is that the database he used contains some major leaks from Korea. 1004 is a fairly popular password there, because it is one of the few 4-digit numbers that sound like actual words in Korean. 1004 sounds like "angel" (
cheonsa).
So if you're actually trying to break into people's accounts, it would be advantageous to know your victims' ethnicity. It's quite likely that cards stolen in Koreatown will have a different distribution of PIN numbers than those stolen in Chinatown.