Well, if one was well read, one could go to the source and define well read.
According to the Oxford English Dictionary well read means
1) Well-informed through reading; having read widely and attentively; learned, erudite.
2) In predicative use. Well-informed in a subject through reading.
3) Of a book, etc.: attentively or (now chiefly) frequently read.
The traditional meaning is the first, well-informed through reading. A well read person is a person who has read or studied a wide variety of subjects and can engage in an informed discussion on those subjects. This is opposed to someone who only has a superficial knowledge, though that rarely stops such a person from loudly expressing their opinion and dismissing anyone who disagrees with them as ignorant.
The second meaning is what the previously mentioned SF writer meant, i.e. someone who is well read in SF is well informed on a wide variety of SF authors and books.
I rarely sees the phrase used in the third sense.