If I say to you, "I have given you your Christmas present," then you already have it. In fact, it has "happened." The happening transpired in the past. The fact of the "happening" is a done deed for the present.
God was speaking here of a future event. He's saying that it is already done even though it is a future thing. It is a completed act in the present, even though for Joshua it was yet future.
That's fascinating.