Hey, who told you that pirate or others dont have assets over their total deposits? They can be absolutely same with banks. They just need time to turn they assets to btc. In some cases this might not be possible in short time, same with banks.
Banks disclose their assets and revenue streams. Ponzi's do not. Moreover, its up to the creditor to prove his creditworthiness, not the other way around. If you go to a bank and ask for a loan, the bank will ask you to prove your creditworthiness through income or assets; you dont tell the bank to prove you are a broke bummer. You could (and probably should) ask the bank to prove they are solvent before depositing there though.
If you dont want your scheme to be labeled as a ponzi, all you have to do is prove a credible revenue stream that can generate those interest rates and prove your assets. If you fail to do both, it should be assumed a ponzi.