Robin used a Dirty Math Trick that works on us because we're not used to dealing with large numbers. He used a large time scale of 12000 years, and assumed exponential growth in wealth at a reasonable rate over that time period. But then for depreciating the value of the wealth due to the fact that the intended recipients might not actually receive it, he used a relatively small linear factor of 1/1000 which seems like it was pulled out of a hat.

It would make more sense to assume that there is some probability every year that the accumulated wealth will be wiped out by civil war, communist takeover, nuclear holocaust, etc etc. Even if this yearly probability were small, applied over a long period of time, it would still counteract the exponential blowup in the value of the wealth. The resulting conclusion would be totally dependent on the probability of calamity: if you use a 0.01% chance of total loss, then you have about a 30% chance of coming out with the big sum mentioned in the article. But if you use a 1% chance, then your likelihood of making it to 12000 years with the money intact is 4e-53.