I'm sorry but you've been misinformed. This article is also factually incorrect in many regards. There are no "tainted" coins. For example this excerpt from your article is completely false:

"Not all Bitcoins are equal and have the same value. Some Bitcoins have been blacklisted and blocked by several entities, making those coins less valuable than the rest. If you receive Bitcoins that were used in the past for illegal purposes, then your Bitcoins could be blacklisted even though you had nothing to do with the illegal activity."

Bitcoins are interchangeable and indistinguishable; exactly alike. They are not physical objects and carry no identifying marks. Their only property is their amount.

> 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.