That’s not what they did at all, it’s just a 3rd claim on the disputed land. They are leveraging the ambiguity on the Serbian-Croatian border caused by the Danube river’s changing flow. They are just trying to exploit a technicality, which is definitely entertaining.

Neither Croatia nor Serbia claim this land. They both claim other, more valuable, nearby lands, leading to this piece of land not being claimed by anyone.

Those are technicalities due to the geography.

As has been pointed out multiple times:

1. Croatia does claim the land: Croatia actively and persistently treats the land as part of Croatia; it e.g. sends Croatian cops to enforce Croatian immigration law and Croatian land use regulations.

2. Croatia signals it would be happy to relinquish its claim to Serbia in exchange for certain other land elsewhere, but that does nothing to invalidate the claim in the here and now. The scammers' invocation of terra nullius is ahistorical and silly; their invocation of bona vacantia is violently dumb.