Income from idle land is certainly unearned. But little idle land can earn anything (grazing land, I suppose, but then you still may have some maintenance or security you provide). Anything else will be earning income not from the land, but things built on the land, grown on the land, etc. A house that I have built (or maintain) isn't land, it's a house. It requires land to sit on. So when I rent it out, what percentage of that income is because of the land and what percentage is from the house? If it's farmland, what percentage of the income is because of the labor and materials put into the land?
Taxes on land certainly make sense, but as written this essay's suggestion is to have a 100% tax on landholders income from land. So how, then, do they make money? Does this only apply to people who rent land but have done nothing to improve it? If I have a family farm that I want to hold onto, but rent out 100 acres to a lumber company does that mean I can't earn anything from that rent because I'm not the one doing the work? This essay needs some work to produce anything approaching a reasonable solution.