What is ironic about the one survivors testimony for Oedipus?

The only survivor's testimony for Oedipus is ironic because it is the truth, but it is also the opposite of what Oedipus believes. Oedipus believes that he killed his father and married his mother, but the survivor reveals that Oedipus was actually raised by his parents and that his father was killed by someone else. This revelation is ironic because it overturns everything that Oedipus believes about himself and his past.