I think he did understand that, in fact he discussed the WEF and the IMF at length in the earlier chapters of his book. I'm familiar with the various schools of monetary theory. Debt owed to humanity, however, is never abstract. There is always a notion of who, or what collection of people, is humanity. This is definitely a subtle point that gets more toward the phenomenology of money.

Did you actually read all of Graeber's book? If you did, I honestly don't think you would make this point.

I agree with you on some of your views of money, but money is a tool. We can do without it if need be and it wouldn't really change much. To do that we need to understand the specific purposes money plays, as well as have some rationalist theory of money that plays nice with our ontology. I don't think that those exist (unless you're a naïve realist).