The point is simple : if he wins he's hoping you'll sign up and pay. That's it, that's all.
Would this be true statement "unless it's an affiliate deal where they pay a percentage of the players losses". Would you be double dipping either way? You are a winner either way can't lose.
I can't argue the point because I don't really know other then second hand information. I was under the assumption that they were paid a % of loses that is why they wanted your account number