Namely, that Moffat was not really suggesting that the TARDIS's swimming pool is generally to be found in the library. Rather, the swimming pool is elsewhere, but with the TARDIS crashed on its side like that, the water from the swimming pool had drained out and pooled in the library.

It seemed pretty obvious to me when he said it that the water had poured into the library... but I didn't recognise the symbolism. That's gorgeous!