But did Harry himself say the address? It doesn't speak well of the enchantment if you can use this "20 Questions" kind of loophole to arrive at the answer anyway ("Is it ____?" "Yes, that's Headquarters"), but I think that scene still falls within bounds. (Lucky for JKR if it does because I bet she wasn't thinking "oh, yeah, Fidelius might limit what people can say here, better be consistent".)