Except, the law is about "reasonable doubt", and has been dealing with this kind of thing for hundreds of years. This isn't new magic.

By a similar argument, any phrase someone says to someone else might have been code for "kill that guy". Except usually it's not, and unless there is convincing evidence you did mean to kill that guy, you'll be set free.

Conversely, if you have previously agreed a code where "2+2=4" means "kill that guy", and then you say "2+2=4", you can't argue in court that "Oh, I was just saying a true statement of maths, how could that possibly be illegal?"