The Eleventh remembered the events only afterwards. The reason he could remember them was that, of the 3 incarnations who took part in the main events, he was the latest. If any of the earlier incarnations had remembered, Eleven would have known all about them before they happened to him. The only later incarnation involved at all was Twelve & his part was very minor, meaning that his foreknowledge had little or no effect on the timestream. If Eleven (or earlier) had had any foreknowledge, that would have caused major effects, which is why the timestream had to be protected by the earlier incarnations having no memory of the events.