Also, I have to ask after reading that again...it seems to be stating that the states do not have the power to create their own currency, I don't see anything about Congress not being able to supply it's own paper bills. i.e. "For though no state can emit bills of credit, or pass any law impairing the obligation of contracts, yet the Congress themselves are under no constitutional restraints on these points." Maybe I'm misunderstanding the wording here, but it seems to say that Congress has the power to emit bills of credit, and I'm not sure I understand why the Federal Reserve would be needed to do that for them.
The basic rule of the Constitution is that any power not expressly delegated to the Federal government is denied them. Or at least that was the original idea. The Fed was created for exactly the purpose you're thinking. The Treasury can't pull money out of the inkwell on their own, so they subcontract it out.