It does serve an important point which is to show again what a proof by absurd is. We found that there is at least one way (impractical or imprecise it doesn't matter at all) to measure something with the oscilloscope, therefore the assertion "it is impossible to measure thd with an oscilloscope" is false, thereby closing a big parenthesis and supporting my argument that if in at least one situation USB cables do make a difference, then the assertion that all USB cables are the same is false.
The only thing you have actually proven, though, is how you can apply proof by absurd. That's fine, but it has no bearing on the reality we will encounter reality. So, at extreme risk of being absurdly logic'd to death...using a scope to "measure" distortion would be an extremely rare case, and actually the misuse of a tool. It's as ridiculous as saying that because you actually can hammer a nail with a wrench you could build a house without a hammer. True in the absolute, but ridiculous.
Perhaps the statement "All USB cables sound the same" is false in the absolute, but it's correct in general and on average in practice. If you're going to live life in the absolute, they you must die. Why? LOGIC! Everyone who drinks water dies! And, sooner or later, everyone must drink water. That means to avoid dying you must stop drinking water, which in turn will cause your death. The same principle applies to a far, far greater extent with prescription drugs. Ever check those side effects? Many include, in the long list, "death". If you then applied absolute logic, you'd never take any of them, even if they could actually extend or save your life.
Wouldn't it be far more beneficial to study the "why" of the anomaly? If 100 different USB cables were tested in a true ABX/DBT, and with a significant number of tests with a significant number of people we came out with a clear statistical fallout that one cable in 100 did, in fact, present a difference in sound quality where no others did, would it be most relevant to study why that one creates a difference rather than to dogmatically declare, "See! The statement that "all USB cables sound the same" is FALSE! One way is research, science and engineering, the other is technically correct, but ridiculous. If you can discover why the one cable is audible, wouldn't you contribute the the collective knowledge of the science of data transmission? The other way you contribute to...well, really nothing other than to declare your own rightness.
Real science recognizes that true absolutes are rare, and many so called absolutes are actually a bit fuzzy. Engineering embraces that fuzzyness and works with it. By clinging to the assertion that the statement "all USB cables are the same" is false, you've embraced the world of the barrister, and left the world of science and engineering behind. You are, as I've said before, both correct and ridiculous.
How do we know, for example, that the USB cables that "sound the best" don't degenerate over time? Is cable rot possible? Sure it is, I've seen it first hand. In USB cables? No, but in cables in general, yes, old cable with cracked insulation, oxidized conductors, discolored outer jacket, etc. So then by your logic you should replace your $1000 rotting USB cable periodically because you can't absolutely say they don't rot!
Can you say, absolutely, that sound itself doesn't cause some physical degeneration? Hearing loss? Death? Nope. So then, stop listening to music! It's sound, and therefore can cause death. How about an equipment failure causing electrocution? You can't even turn you system off without risking that!
Aren't you worried about how many people get struck by lightning? Or killed by meteors?
Can you say, absolutely, that posting in a forum cannot result in physical degeneration such as RSI as one example? How about some other sickness? Death? Think carefully now, and take appropriate action.