What is the factual difference? The similarities being that both rely on finding an ever greater number of people to buy in so that early investors can cash out.
That's a pyramid scheme, a Ponzi scheme is a subset where earlier investors are paid profit using money from the later investors, giving the appearance of a functioning business.
In the same way you can argue that investing in gold is a pyramid scheme (since it relies on its price going up), you can argue that crypto is too.
(Note that there are some real Ponzi schemes in crypto too, but they're not real cryptocurrencies either.)
