As you say, both saying 'we just got a warrant' and not including the warrant canary language would be against that law.
However, you don't neccessarily have to obey all laws - a law can prohibit you from making a statement; but a law cannot compel you to make a statement - so the claim is that if your actions conflict with the law in this way, then (at that instance) application of this law is unconstitutional and your violation is acceptable.
But why couldn't it? All the executive orders so far have been pretty draconian. What would exactly stop them from explicitly stating that Apple is not to signal everyone using their canary and to leave it there. Maybe even providing an assuring immunity that they will be not prosecuted for making false statements in company's reports.
It would likely violate the First Amendment. See, e.g., Wooley v. Maynard, 430 U.S. 705 (1977), holding that a state cannot compel an individual to display the state motto on his vehicle's license plate.
A canary involves someone removing or withdrawing a statement they made previously, it is that action that the gag order would prohibit.
Now personally I hope Apple succeeds and other companies do the same.
Simply, when they are making the next PR statement, then that next time they won't include a specific claim 'No warrants yet', and leave people to interpret as they want.
If the report is written from scratch every time, there's no need to remove the canary statement. You just don't include it in the next copy of the report.
The old statement is not changed or withdrawn, the new report just doesn't include the text.
The law doesn't require you to do anything but not reveal the gag order. In such a situation your actions have caused you to be placed in a situation where you either have to lie or violate the law and the judge will recognize your intention was to reveal the gag order.
I'm not sure why the warrant canary line of reasoning doesn't extend to all situations where you are legally required not to reveal something. You could exhaustively enumerate every possibility (I did not receive a warrant asking about cash transactions in zip code 76225 targeting an occupant on maple street as a result of an ongoing FBI investigation).
You're not going around saying 'wink wink nudge nudge I didn't get a 214 order and I didn't get a 216 order, guess what I got'. The canary has no specific information from the gag order, and is incapable of having specific information from the gag order.