The External Test looks at other historical materials to determine if they confirm or deny statements within the manuscript, i.e., archaeological evidence. The New Testament fails this test, as well. If he did, then he either meant what evangelical Christians think he did, or he didn't. If he meant what evangelical Christians think he did, then he was either a liar, a lunatic, or lord. The financial debt analogy is a poor one.
Moral wrongdoing is quite different from financial debt.
But the better analogy destroys the case for Christianity, because justice is not served by one person serving the criminal penalty of another. People can only be punished for their own crimes. All of the arguments in this appendix are awful. For example, if I say there is nothing but a table in the room, then the table is the room and any table as we know it would be unidentifiable. The universe is, by one common definition, all that exists.