You'll have to mail Marilyn Gardner if you want to know what she meant by "acting white" since she tosses the quote out there without attributing it to any paticular person (They) and doesn't elaborate.
For too many folks "acting white" DOES mean speaking in an educated manner and even being courteous to white folk. An ex-colleauge of mine (female) was always chastised in high school for "acting white" because she took advanced placement classed.
For a more academic discussion of the subject, see the works of Professor Orlando Patterson.