I don't have much to say on this, most has already been said. Here's the text of the speech, here's a comment by a cardinal, and here's a good commentary by an American priest.
Some commentaries have said that perhaps the Pope somehow didn't realize that the quoted statement of Emperor Manuel II Paleologus seemed offensive. I don't think that's the case at all, according to the text of the whole document that is why he used it: "he addresses his interlocutor with a startling brusqueness, a brusqueness which leaves us astounded" and "after having expressed himself so forcefully. . ." The Pope realized that this statement was incredibly harsh, which is why it stood out to him in his reading, and why he used it to launch these reflections. I think that what the Pope did not realize is that this statement, a quotation from a medieval dialogue, would be broadcast in the media as though it were his statement, instead of being read and presented in its context.