I agree, this guy had to know the Ramsey's even if they didn't know him.
If the Boulder authorities turn up solid evidence against Karr, I would agree with your sentence word for word. Otherwise, what I would say is the killer had to know the Ramseys and the house even if they didn't know him.
If the killer is a total stranger to the family and that house, and he picked JonBenet randomly, then the circumstances of the killing -- as they've been related to the general public -- are one long series of amazing coincidences.
This guy is shown that he's obsessed. It's possible that he is the killer and this obsession started before her death. He could very well have gained information about the family. He could even have sneaked into the house on more than one occasion. He might have been familiar and comfortable there. It's all speculation until we get the DNA results.