Yeah, these "river finds" are a bit of joke, but if enough people believe just about anything, well, that thing they believe in becomes true.
Culturally I think the significance is helping a few people to stay alive a bit longer, not all that easy to earn enough to keep body & soul together in some places.
But on the subject of shonkdonkery in Indonesia, it is enough to say that if something can be faked, that can be done in Indonesia.
This link has nothing at all to do with keris, but it is only a short article & it is worth reading through.
http://www.ming-wrecks.com/Fakes.html
I have known several people, not Indonesians, but living in Central Jawa, who lived very comfortably for many years by sending Indonesian faked items --- mostly porcelain, & often with barnacles attached --- to auction rooms & dealers all over the world.