DeLillo - Born 1936, published first short story in 1960, novel in 1971
McCarthy - Born 1933, first short story published 1959, novel 1965
Roth - Born 1933, won award for "Goodbye, Columbus" in 1959
Updike - Born 1932, first published in 50's and 60s
Morrison - Born 1931, first published in 1970
Mailer - Born 1923, first published in 1949
Remember that 50 years ago (from the time this was written) is 1965. Morrison is the only author you mentioned that doesn't have published works from before 50 years ago, and even then she still belongs to that same generation.
King wasn't trying to slight any of them. It's just really has been a very long time.
Edit: To compare, Tartt was born in 1963 and Franzen in 1959. I think this is really the generation he is referring to.