I think there are basically two plausible scenarios, either they received a NSL or they do no longer believe that a canary is necessary and misjudged the interpretation. (In the second case, they could of course clarify in the comments.)
It's a dead man's switch. If it's gone, they've been served with an NSL and the site has been compromised. There's no room for interpretation and no other scenario can be considered plausible.
