It's a scam because he says "You will be able to earn bitcoins and double them" which isn't true.
Where does it state that? Not on the site...
Most people will lose.
You're definitely right about that, because the OP is honest enough to admit that it is a ponzi scheme.
On a gambling site you know that in the long run the house always wins and you will lose, does it make gambling sites a scam? No, only if they would rig the games.
BTC-flow doesn't rig the game and is open about the structure, so everyone knows what's going to happen. The end result isn't pretty, but that doesn't make it a scam imo.