What does duck-quacking mean?

This is a reference to the (humorous or ironic) duck idiom: If it looks like a duck, walks like a duck, quacks like a duck, it is a duck.

used to say that something is probably exactly what it seems to be and we should trust our judgment about it:

They're calling it a clinic, not a prison, but if it looks like a duck and swims like a duck, then it is a duck, I think.

In the same manner, the author claims that the business model of DFS is that of parimutuel pool of racetracks and even casino gambling, because it operates in the same manner. It's a duck because it quacks like one.

The author of this article is relying on his reader being familiar with an old expression: If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

In an implied parallel, the author is asserting that since Daily Fantasy Sports looks like casino gambling, and does things that casino gambling does, then it must be equivalent to casino gambling.