He seems to be pointing out that pluralism is consistent with moral objectivity (and so does not imply anything like relativism). The defender of moral objectivity need not deny that societies believe different things about the nature of morality and that our moral behaviors and beliefs largely have to do in the culture we grow up in.
That is to say that we can have socially determined value opinions without the possibility of objective values being undermined.
Begging the question is the fallacy of circular reasoning. Your interlocutor “begs the question” when he cannot arrive upon his conclusion without assuming that his conclusion is true, that your conclusion is false, or something similar (ie. using a fact which is epistemically posterior to his conclusion).
In this case, it seems that Kreeft is saying that those defending relativism (I’m assuming) on the basis of pluralism and the existence of different socially determined value opinions beg the question to claim that this implies moral relativism or the nonexistence of objective moral values.