The statement "in all forms of gambling the gamblers make a net loss" is ambiguous as to whether it's the gamblers as a whole or each of the gamblers making a net loss, and while I grant that it reads more like the way you're describing, the later context pushes the intended meaning in the other direction.

He says "The 'house' is the only winner," which is as a point of fact untrue - there are also individual gamblers who are winners. Saying this implies he thinks that each of the gamblers is a net loser.