Probably most important is psychological one since there was a war and violent clash between disputed sides in the 1990s and that land was claimed in very similar manner like this guy is attempting now. Coming there with such claims is very wrong and locals and authorities from both sides will be irritated by such attempts.
This guy's sad interpretation of current dispute situation would be something like: there is a free piece of land and everyone is invited to claim it. That is far away from reality. There is a dispute about that area but that doesn't mean that it doesn't belong to anyone and that it is not controlled. At the moment land part access is controlled by Croatian police and river access by both sides. Legally that land still belongs to Serbia and that comes from last legal border agreement between two sides which is constitution document of Yugoslavia dating from 1974. Borders between countries were defined back then very precisely but that document is subject of dispute from 1990s till now. Dispute and border changes were caused by war, politics and riverbed changes.
Both river and land were used for years by locals as a result of agreement between disputed sides in order enable local population to access that area without too much hassle by the border police.
Now, thanks to this idiot, locals are banned from using it any more. Besides collecting a lot of money for his agenda from similar people like him around the world, that is the only concrete result of his actions.