Re-reading the original Joel test, it is clear that Joel means quiet in the sense of free from interruptions.

There has been an ongoing debate on the benefits of having developers in separate offices for many years versus have developers in an open plan office. There are arguments in favour of both sides. Joel, however, has plumped for having developers in their own offices.

So in this case I believe that Joel's (opinionated) views remain as valid now as they did when he wrote them. After all, even though there have been improvements in noise cancelling headsets (and office etiquette now allows wearing them), it doesn't stop interruptions from other developers.