This is a fair point, and I see the argument about the state as a real bugaboo for Graeber. I know it figures centrally into the argument about how credit gets created as an instrument. But I think you're missing the point (and maybe the entire thread is) by focusing only on what he's saying about credit. Because although credit as an instrument has evolved in striking ways throughout this time, his argument is only partially about credit: it is also about debt, which is a more metaphysical, moral concern that, strikingly enough, remains deeply embedded in our general thinking about how money and credit can and should work.
And, in a sense, it is the presumed immutability of this concept in current discourse that he's looking to undermine, i.e. "People must repay their debts" is an idea that has often seemed completely immoral at many points in the history of credit, money, etc. It's worth investigating how this has changed, why it has changed, and why, right now, it can seem "right" (and not just legal) to force unemployed students into debt peonage or homeowners into foreclosure for underwater mortgages. As I read Graeber, his point is that there has been a striking diversity in the way people think of both credit and debts (as well as how it has operated).
The question is why we haven't yet seen the kind of reaction one would think you'd see when a tiny sliver of the population holds the rest in a form of credit/debt slavery. It isn't fair to say his goal is to look at some arcane, incidental past and make a direct analogy to today or project some Mesopotamian system forward to Goldman Sachs (though I'd love to hear Matt Taibbi attempt to do this). His point is rather that, in other specific historic situations people have had very different reactions to how debt, credit, and exchange were understood. This should help encourage us to understand the way the particular kinds of debt we have today are being considered in the way they are being considered. In other words, whatever anarchist bias is sneaking into his understanding of the patterns in the creation of credit, it seems less critical to the value of the project than his anthropologist bias towards looking at people's varied, concrete reactions to debt. I realize those are connected, but both are a helpful corrective to the Robinsonade fallacies he's targeting.
sean