I see DRY as a smell, not a principle. If you see clones (same code in multiple places), then it's likely indicating that there is something that can be factorized. Now the question you should ask yourself before factorization is whether the duplication is coincidental (as the author shows) or if it's because the logic was copy pasted. Most of the time it's the second case, and duplicate code does make maintenance harder and riskier.

One of my pet-peeves is clones in unit-tests. People tend to care less about code quality when it comes to unit-tests, and code gets copy-pasted all over the place. The result is usually an unmaintainable ball of mess, where the most subtle variation in the unit being tested requires you to apply the same change in 15 different places. In this situation, DRY is a very useful indicator that something is going wrong.

Now the opposite of DRY is YSHRY - You Should Have Repeated Yourself. When you start adding 5 boolean parameters to a function to adapt it to all its calls, it's a smell that you thought you should have DRY, whereas YSHRY.