That part - "Rome was not only white" is the motte from which the author is pushing his ideas about "whiteness", "colonial bias", "diversity in classics" and more such things. In short, he is trying to inject critical theory into the classics by attempting to racialise the subject area. The fact of Rome being "white" is also inconsequential since the classics tend not to be seen in a racialised light other than by people like this author and other racially focused individuals. Should they - he is not the only author who attempts this - succeed in racialising the classics they'll use these as yet another weapon in their attempt to deconstruct western culture.