Not really. It actually is unclaimed land - the border between Croatia and Serbia was drawn along the middle of a river. The river moved. One country (can't recall which) claims the border is still mid-river, the other claims it follows the original river line, not the new position. This has created areas of overlap and areas of terra nullius - there are a few tiny ones, much smaller than Liberland.

And if no-one claims the area, you can wander in and stake your own claim. Of course, if no-one recognises you, there's not much you can do, and if you don't ally with someone meaty, no-one will stop an 'annexation'. Ultimately it's a jape, but unlike most micronations, it's not stretching the rules - the land really is currently, officially, unclaimed.

Vit Jedlicka is actually all out conspiracy theory blog author (reformy.cz), so not really prank.