> They are not physical objects and carry no identifying marks. Their only property is their amount. True. It's not the "bitcoin" itself that's being tracked

> They are not physical objects and carry no identifying marks. Their only property is their amount. True. It's not the "bitcoin" itself that's being tracked and tainted, it's the transactions between addresses that are being linked. An amount of Bitcoin is just the balance of an address. However a property of Bitcoin is that transactions contain the plaintext address where an amount of Bitcoin was deducted and the plaintext address where the corresponding amount of Bitcoin was credited. Thus you can track an amount of Bitcoin as it was held by different addresses. While it's intangible, it is quantifiable and traceable.