How rude can Socrates be? A note on Phaedrus 228a5-b6

In Phaedrus 228a5-6, Socrates recollects what Phaedrus has just said about his meeting with Lysias. In this passage, 228b5 εἰ μὴ πάνυ τι ἦν μακρός is traditionally read as an independent sentence, saying that Phaedrus is able to learn by heart a discourse unless it is not a bit too long. So read it would make Socrates behave in a rude way, as he gratuitously reminds us that Phaedrus’ ability to retain discourses is pretty limited. But when εἰ μή is governed by a swearing such as 228b4 νὴ τὸν κύνα, it has emphatic force, meaning, in this case, that Lysias’ discourse was in fact a bit too long. When the sentence is read according to this syntactical device, Socrates is not at all rude towards his friend Phaedrus; he is just teasing him and mocking rhetorical techniques in passing.