Here is that statement from Scotland Yard:
"The prime minister has been interviewed briefly to clarify points emerging from the ongoing investigation. He was interviewed as a witness, not as a suspect and co-operated fully.
We requested the meeting was kept confidential for operational reasons. We are not prepared to discuss further."
So we still don't know the reason for the police's request for confidentiality. Could it have been to avoid tipping off Lord Levy who the police questioned four days later? On the other hand, the interview with Levy was fixed with his agreement so this explanation would only hold if the police booked the appointment the day before they arrested him.
PS. What it does show is that for all the talk of leaks, both the police and the politicians kept this one firmly under their hat.