It's not a Ponzi scheme because in ponzi scheme you deliberately use late investors money to pay early investors.
What happens in cryptocurrency is fake-exchange scheme. Tether is a fake currency printed in unlimited way by bitfinex which is also an exchange. By pretending tethers are real dollars you create the appearance of a stronger market for cryptocurrency than actually exists. It all rests on increasing the number of tether in circulation. It will fail when there is a run-on-the-bank on tether.