my thought process is: one large BTC pool. all transactions are feed into the pool. the order of the deposits dictate priority of withdrawal. Owner does not skim the transactions, only makes a large initial deposit and then markets the site like crazy. Big assumption here is that he is running an "honest" ponzi operation.

why would he even need an initial deposit? he can simply delay to pay people until enough other people have paid.

So that he can "honestly" take the 20% additional of that first deposit.