Of course, that note is for copper cables. The longer lengths, as from Corning, are optical fiber, which, as you know, is much more expensive. Maybe a shorter cable, less than 10m, is not practical to manufacture from glass fiber. That is, a 5 meter optical cable might have to sell for nearly as much as a 10 meter length.
One advantage to the copper cables is that they can carry power, where the optical cables only transmit signal - no power - to external devices.
As all cables must be certified, and the Thunderbolt spec limits copper cables to 3M, your choices are limited.
Bit the bullet and spend the money for optical, or move your system closer together
The copper cables do have logic on each end so it is not simply a plain copper cable with an mDP plug on each end. The optical cable is even a bit more complicated, hence the cost.
Just pretend that the optical cable is a high performance "Monster" cable.
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One advantage to the copper cables is that they can carry power, where the optical cables only transmit signal - no power - to external devices.
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For something that is being placed far enough away to need optical cables probably needs more power than Thunderbolt would supply over a copper cable.
And besides the longer distance, there is an advantage to the optical cables. They electrically isolate the two devices so if one has a power surge, the other device is not affected over the optical thunderbolt cable.