That’s one of the issues with the quote. It’s commonly cited as 2002 in the G&M , but it is (was — the page is no longer visible) also quoted in a 1993 book. Galbraith using the core phrase (“superior moral justification for selfishness”) can also be found in a 1998 book, another 1998 book, etc.

The bottom line seems to be that Galbraith used variants of the quote over the years, but the 2002 version was the one that was most visible when the quote “took off” (perhaps on his death in 2006) and so was used as the common citation.