When people were investing a year's salary buying tulips back in 1636 in the Netherlands (google "tulip mania"), it wasn't because of a scheme. It was because they overestimated the lifespan of the market for tulips at the current market prices. The driving force of this phenomenon wasn't malfeasance but the greed and stupidity of the market participants.

The distinguishing characteristic is that a Ponzi scheme entails a fraudulent claim of investment by a fiduciary. A person telling people they are investing in bitcoin through their brokerage, but who is actually paying out the investor's capital to newer investers or otherwise misallocating it is involved in a Ponzi scheme, strictly speaking.

But most of the investment driving bitcoin's valuation is legitimate, in the sense that most people actually own the bitcoins they think they do. This can't be accurately described as their being involved in a Ponzi scheme.

Cliffs: There is no good reason for muddying definitions by describing the commodity bubble of bitcoin as a Ponzi scheme.