coinmill.com says that they use current exchange rates. With an inflation rate like that, you don't want to sneeze while selling ZWD as it will cost money.
I'll bet it's literally not worth the paper it's printed on.
Edit:
According to coinmill.com, 1 USD == 12,625,549,717.8 ZWD so
100000000000 ZWD == 7.92 USD
So I'm wrong, it is worth the paper it's printed on, barely.