"I have never seen Dr. Strangelove", by Ron Rosenbaum.

Seriously, the letter seems to be intended as a weird low-tech version of the automatic triggering mechanism required to resolve the paradox he's so puzzled by. You could even see it as an improvement, sort of: if the contents of the letter are like a 50/50 coinflip, then it's almost as deterring as a 100% trigger, but with a chance of /not/ committing belated genocide. That would also explain why they made it public.