Indeed, to assume from silence that Paul did not know the Jesus tradition because he does not cite it more explicitly and more often is almost analogous to assuming that the writer of 1 John was unaware of the Johannine Jesus tradition because the document presupposes rather than cites that tradition.
Craig Keener, The Historical Jesus of the Gospels
To me, this is just typical apologetic claptrap. Keener accuses the skeptic of assuming that Paul doesn't know about the Jesus tradition (whereas I would argue that the skeptic infers it as the best explanation for Paul's silence). But what does Keener offer in response? A presupposition! There is something wrong with concluding that Paul doesn't know things that he doesn't mention, but it is apparently perfectly reasonable for Keener to affirm Paul's knowledge of things that he doesn't mention based on presupposition.
I don't see any reason to view Keener's statement (and McGrath's quotation) as anything more than a smokescreen. Even if I cannot infer Paul's lack of knowledge, I am still left with no evidence of what Paul knew about things that he does not mention, which leaves most of the historical core uncorroborated by the earliest source. I am still left with Mark as the earliest source for the traditions.