IIRC, the President signed the treaty, but the Senate never ratified it.
Further, the perp never brought up his alien status during the trial(s). It only became an issue AFTER he was sentenced.
So to hell with the treaty, and the author.
I wondered about that myself. Also, if you can’t ask them about their legal status when you arrest them, how are you supposed to know you have to notify the Mexican Consulate about them?