Earlier quoted context omitted.
You get transitive credit from direct credit. Alice wants to buy an apple from Charlie. Alice gives an IOU to Bob, Bob gives an IOU to Charlie, Charlie gives the apple to Alice. And - sure, everything crypto does could be done with pen and paper and a phone line. But here Bob's computer can agree to do this without Bob's involvement.
This only works at a minuscule scale, where people could actually get to meet face-to-face. On a global network, Alice can still create an attack against Charlie. She just needs to get more intermediaries. Even if the intermediaries thought "surely Alice won't put the relationship at risk over a small amount", no one in their right mind would be doing large scale, automated transactions based on "friend-of-a-friend"…
What is the scenario you're imagining of Alice attacking Charlie?