I think in his world, his "right" to own the land comes from his ability to defend what he thinks he owns. However, as I've pointed out in a limitless number of threads, this quickly devolves into the man with the biggest gun making all the rules.
What I find funny is exactly what you've written here. The pro-libertarians here seem displeased that we all have to obey the state - "might makes right" is a terrible principle, they expound. However, as you say, they'll then argue that you can only own what you can defend. Hmmm.