Knowledge by Acquaintance

I was led to the following interesting remark by a review in today's WSJ:

There are characters whom we know to be historical but of whom we do not feel that we have any personal knowledge -- knowledge by acquaintance; such are Alexander, Attila, or William of Orange. There are others who make no claim to historical reality but whom, none the less, we know as we know real people: Falstaff, Uncle Toby, Mr. Pickwick. But there are only three characters who, claiming the first sort of reality, also actually have the second. And surely everyone knows who they are: Plato's Socrates, the Jesus of the Gospel, and Boswell's Johnson.
--C.S. Lewis, "Modern Theology and Biblical Criticism".

Lewis goes on to say that our sense of the character of Jesus or Johnson is so strong, that our judgments coincide on the spuriousness of apocryphal gospels and 'pseudo-Johnsoniana':

Our acquaintance with them shows itself in a dozen ways. When we look into the Apocryphal gospels, we find ourselves constantly saying of this or that logion, 'No. It's a fine saying, but not His. That wasn't how He talked.' -- just as we do with all pseudo-Johnsoniana.

But curiously Lewis (regardless of whether he is correct about Jesus and Johnson) is silent on whether this test works also for Socrates. Do the sayings of Socrates in Xenophon, for instance, ring true? If Lewis is correct about Plato's character, we should at once agree either that 'That's how he talked' or not.