A Ponzi pays out profits to early investors by using the money of new investors. New investors, are promised the same profits, but the system eventually cannot pay it out when outflow overcomes inflow. A ponzi is managed by a central system that determines who gets paid, that processes profits.

There are no promises in trading Bitcoin, except for the ones that investors imagine for themselves. There are no benefits for recruting newcomers, you don't earn referral fees for new people buying Bitcoin. There is no central scheme in Bitcoin, traders decide between themselves what price the buyer is willing to pay to the seller. For as long as someone is willing to pay the seller's price, a trade happens. There is no exchange of new or old money, there is exchange of existing value.

If Bitcoin looks like a ponzi scheme, then in general, the trading of every stock, commodity and asset can be seen as a ponzi.