People call all sorts of things a Ponzi Scheme but that doesn't make it so.
All invented currencies reduce to Ponzi schemes unless there is an active community exchanging value with each other. People actively use the US dollar currency to exchange value, but if they stopped using the US Dollar then it would collapse into a Ponzi scheme. The same may or may not hold for any particular crypto-currency. Check the whether or not the community is exchanging value using that crypto.