I've searched for this interview quote before, and I've come to the conclusion that it has all the hallmarks of an urban legend. Everybody is sure they've heard it somewhere, but nobody can trace a source. I think it's probably a conflation of two separate facts JKR gave us in older interviews: 1) we'd see Lupin again in Book Five but not in Book Four, and 2) we'd find out something about Lily in Book Five, and something tremendously important about her in Book Seven.

I suppose there's a slight possibility that she said it before the books took off like wildfire and people started recording and memorizing her every word, but it would have to have been after the release of PoA, which leaves a very narrow window of time.