Movie Logic - Can Writing Instead of Saying Beat a Gag Covenant?

I was watching a Nigeria movie where a group of criminals killed a man and in other to keep this murder secret, the leader of the gang made every member take a blood covenant promising not to reveal what happened to anyone. If they dare attempt to, they will be struck with whooping cough and subsequently choke to death.

Some way or another, the police was able to get on their trail and arrested a few members of this gang. During interrogation, anytime they try to mutter a word about the assassination, they immediately started coughing and drop dead.

So the movie plot had me thinking. What will happen if those trying to confess write their confession on a piece of paper instead of saying it, will the Juju/Vodu still catch them.

The Vodu/Juju instruction says no third ear must hear the secret between two parties, it does not say no eyes must read the secret between two parties.

So, if they say you will die if you tell anyone, just write it, you will safe. Just saying (Raise hands and walk out of the room).