Somebody is looking after it as there has been logging activity over the years. Chances are that he just told them he had permission to be there. It also could be that Mr Giles knew about it, but once faced with environmental concerns he very conveniently forgot.
I understand your comment about adverse possession. However, while he has been there since 1998 the landowner has been trying to evict him for 6 of those years; so the required 20 years hasn't elapsed. In any case I don't believe that he would have been able to claim the entire 73 acres... probably it would just be the land which he actually occupied. He's been there for free, has apparently been stealing wood for years, and his story keeps changing. Based on some of his comments, it doesn't sound like he's the "Nice old man" which some purport him to be.
This string of articles just goes to show how the media's spin on a situation can really affect the public viewpoint.