The Doctor just let her go on a minute, because if he was strictly honest with himself, that did sound rather charming. "Well, no. The Elixir of Life is something the Sisters used to use to extend life spans. We'd use it occasionally if a regeneration or something was going wrong. I'm hoping the hot springs are still here."
That's where it was made. Shouldn't matter if no one was on the planet. He was counting on it, actually.