This.  In a Ponzi scheme, you pay money to someone, and they pay you a return.  But that return that they pay you is actually stolen from someone else.

Bitcoin is just like a stock or a commodity.  You buy a piece of a limited resource, or you're given a piece of that resource, and then you re-sell your piece later.  If you pay $1000 for a Bitcoin and you're only able to re-sell it for $100, that might make it a bubble, but it doesn't make it a pyramid.