I think most of the people use this word to refer to any kind of activity similar to this:

Early adopters of a certain asset attract more and more people to join the game by a fast price appreciation and sell the over-valued assets to them later

In this case, he might refer to bitcoin early adopters who accumlated large amount of coins pump up the exchange price and sell them to late adopters to profit

But there is one thing he missed: Any kind of such scheme is rely on the supply of fiat money, so typically when FED tightens and fiat money supply becomes less, this kind of scheme will collapse. But bitcoin itself is money, it can exist without fiat money, as long as you use it to buy and sell goods/services

Anyway, the early adopters' benefit is very clear due to current design of halved coin supply every 4 years, this is the main reason that people relate bitcoin to a ponzi scheme, and also the reason that many people want to make alt-currencies to become an early adopter. If bitcoin protocol defined a different supply speed of coin (for example daily coin supply only reduce 33% or 25% every 4 years), then this kind of complain will be less. But then people might not generate enough interest in it so quickly and it would not have that kind of degree of price appreciation

Even if bitcoin are a ponzi scheme, due to its limited supply, this ponzi scheme might last hundreds of years