Board Thread:The Panopticon/@comment-4028641-20151011010956/@comment-26469787-20151013113202

Danniesen wrote: If this explanation is for Clara, which is much likely (almost bound to be), then it will have to happen after the events of this episode as she's not in the TARDIS before those events.

I mean at some unknown point in their earlier travels when the Doctor chose to explain the concept of the bootstrap paradox. Therefore, he could be alluding to an earlier explanation of similar events with his final line.