I'm pretty sure the "myth" of barter is really just an educational/explanatory crutch that economists have used to introduce laypersons to the idea of money without overwhelming them with caveats and nuances. Kind of like how schoolchildren learn that the American Revolution was a revolt against unjust "taxation without representation".

You can tell that Graeber isn't an economist because he mistakes the oversimplified educational narrative for what monetary economists actually believe about the history of money and credit. But you have a fair point--even economists often fool themselves about this, possibly because the field has grown to the point where many aren't as familiar with this topic as they should be (I don't think that was the case 100 years ago).