It's not. They explain that a stamp DID have intrinsic value, UNTIL (if you read on they explain) the credit of the USPS had been taken away by policy that a smudged/circulated stamp is invalid
I think the original authors point is, that the US Postal Service's performance of letter delivery, is not a denominated currency like ounces of gold or cents and dollars. But the real value of a stamp would be ounces of mail, and not only the act of delivery
This seems to be a contradiction. The stamp is a demand upon the credit of the US Postal Service to perform an act of value. What is the difference between that and a piece of paper which is a demand upon the credit of a bank to perform the act of handing you silver?