I think he's saying that if someone came into your house and said it was theirs, and had the ability to enforce that, it wouldn't be your house anymore.  No, you can't <i>ethically</i> say "I claim this piece of land for my own. Oh, look! There are people living on it already. I guess I get to enslave, rape, torture them."  But if ethics are not of any concern to you, then you can certainly say it and do it, and the land will then be yours.