I totally could see someone mentioning it to him while he was touring DA prior to the grand open. Who knows how they heard about it, or if they were just guessing after seeing that door open and the fact that you can see Fear Factor through it? :shrug:
I think a magical fireplace in an otherwise boring London office facade would be a great entrance personally. But we do have the entrance to Leaky Cauldron inside Diagon, even though that should be in London, so anything's possible I guess.