Or if they said "you will double your coins if you play here". They don't say that. If OP didn't say that, he wouldn't be running a scam either. But he does, so he is.
You're wrong, the OP says:
BTC-Flow is not merely a fascinating game. You will be able to earn bitcoins and double them in a matter of days.
Small nuance, but it's is not the same what you claim...
Anyway, I understand there can be a difference in opinion about ponzi being scam or not.

There's no real difference between "you will" and "you will be able to", and neither is true for everyone. His statement is still untrue for some values of "you".
Anyway, I understand there can be a difference in opinion about ponzi being scam or not.

I don't think all Ponzi's are scams. But this one is, since he claims everyone is able to double their coins while is clearly not true. It's the deception that makes it a scam, not the fact that it's a Ponzi.
Just to add, if people are investing here that means they are ready to loose and gain both, which is same with all whether it is a ponzi or gambling site.