I think the point was that killing a Serbian military officer would start a war with Serbia. Of course, the challenging part would be convincing Serbia to back the scheme; if Serbia was going to defend the territory they'd be more likely to claim it themselves.

Serbia has no intention to claim this territory whatsoever. Serbia treats the Danube as the border and that is it. The new Danube stream that is, the one now dividing the two countries. There is an old Danube stream which is almost completely gone decades ago, and Croatia considers it its border. The reason is that there are some other inversely interweaving parts which are pretty and maybe a nice tourist attraction and both countries want them. So, accepting Liberland as their own territory would mean waving the right to those other, more profitable, territories. But TBH, using the old stream as the border would just cause more trouble.

To be fair, I'm a Serb and I fully welcome Liberland. Since the two neighbors cannot decide on their boundaries, I do not see why anyone else cannot take a no-mans land. The issue is that we must define our borders sooner or later, and I am afraid there will be no more space for Liberland. But, that will be Croatia's business then, I hope :D .

Neither Serbia nor Croatia claim the land because they both -- under different interpretations of the underlying agreement -- recognize it as land belonging to the other under an agreement between the two of them.

This also makes it unlikely that either would act as a guarantor of the independence of a new sovereignty that sought to establish itself on the land, or even to recognize such a sovereignty.