No, it isn't. Just because somebody pirated it does not mean it is open source.
Yes it is, the sources are now easily available to anyone who bother to look for them. Whether sources are "open" or not is not a matter of legality, it is a matter of logistics. In most of the world, it is not illegal to be in possession of the sources, or to do whatever you want with them. Hence de-facto open source.
If I would rob your home, would you agree that all your belongings are mine just because I could break in?
Flawed analogy. If you come to my home and copy everything I got, I honestly would not mind, even when you copy work I had spent years on.