Please note that the legislation you are referencing is completely unrelated to the Assistance and Access Act. It's talking about journalistic warrants (warrants on journalists not by journalists) which are their own shitstorm, but a very different topic.

The Assistance and Access Act does have the same "existence or nonexistence" wording as the law you referred to, so you're not wrong that a warrant canary for a notice under the new legislation would be a criminal offense. But there is no blanket law against warrant canaries per se, it's done explicitly in each new law about secret warrants.