It would be an interesting legal case to say the least, especially because they do not indicate that they will provide further information about the warrant, or how to act other than individuals should "take special notice".
Still I believe you are correct, by having a publicly stated policy about their planned inaction when served a secret warranty is effectively the same as making a public announcement. Ballsy, but kudos to them for making a statement about their privacy beliefs.
It does seem like something that only works once. As soon as a warrant is served and they stop updating the file, when can they start updating it again? When the warrant is made public?
Does it matter? Either rsync.net is clean, or it's not. Once surveillance begins it could continue indefinitely, and once law enforcement decides rsync.net is a threat it's likely that they'll become a regular target. After that point, if you're not OK with having your data searched by government agencies, you're not going to be entirely comfortable with rsync.net. But until that point, rsync.net can stake a big claim on keeping your data safe.
Still I believe you are correct, by having a publicly stated policy about their planned inaction when served a secret warranty is effectively the same as making a public announcement. Ballsy, but kudos to them for making a statement about their privacy beliefs.