There have been indeed some declarations from the writer in regards to Far Cry 3 ending, and the whole story for that matter. More on that below.
In regards to the ending...
No, there is no clear explanation as to what happens in the end. Nothing in the credits, nothing at all except Jason words. You are supposed to interpret them in the same way you have been interpreting the whole game.
"And how should i be doing that"? Well according the Jeffrey Yohalem, the game writer, you should have always kept in mind that you are playing through Jason eyes. All the over the top action, the setting, the high-class jobless hero who hasn't grown up, the island itself and the inhabitants. It's all an exaggerated depiction of what people fantasize about when they think "video game hero".
As you progress in the game you find yourself in specially weird situations where there's no clear explanation of what is happening. Buck's fight dance. Vaas killing and finally Hoyt's killing. At times it becomes surreal, and you don't know what really happened, you just know the results of that.
Have you notice, for example the Lewis Carroll quotes at he beginning of each chapter? Yohalem said that players are supposed to relate these quotes to the events of such chapter. And how what you are playing might be just what Jason is perceiving, and not the reality of Jason. All the action partially being delusions of Jason about his current situation. It kinda makes sense as we know that Jason is pretty much the less useful of his entire group of friends. How is he doing all of that?
So in conclusion,
The ending, and the game as a whole must be considered as a perspective of Jason's reality, and not his actual reality. So we don't know for certain if he escaped the island or not. Considering what was stated above, one could say that he did escape, but the reality he escaped from is not of what we see, but of what he perceives. For all we know all those adventures might have been greatly exaggerated, and he barely managed to escape. Also in the dialog with his friends one can start to see glimpses of insanity and psychosis in Jason psyche, so we can trust his judgment even less.
With all that said one must note that this is the writers intent as he has explained in some interviews, for example here and here here. And we, as player must remember that one thing is what the witter wants, and another what the game direction wants. Because you might have probably notice that some of this stuff seems shoe-horned into a "shoot and wreak mayhem" game.