Maybe it's buried in footnotes to that page or in their ToS or something.

In my mind, the "correct" way to do this is to:

1) pgp sign the entire warrant canary statement with a published key.

2) add unfakeable news headlines to the canary itself (such as recent stock prices or sports scores)

3) date the warrant canary and include a well defined schedule of updates

I believe this is the authoritative example:

https://www.rsync.net/resources/notices/canary.txt

Other then the scheduling piece, this seems like the kind of problem a block chain would _actually solve_. I know we're all tired of "The banks/shipping industry/insurance need to use webscale blockchain!!!!"

But a signed message that's publicly audited as authentic and stored non-centrally seems perfect for canaries....