He did the same trick as I did to test my faulty PCIe extender if my board atleast supported PCIe Bifurcation. FurMark doesn't transfer much data over the PCIe lane: a handful of textures and very little CPU load. That's why people recommend running Prime95 along with FurMark, because FurMark only stresses the GPU and is really not doing much between the CPU and GPU.
So he basically tested at what length you could run FurMark over Thermaltake's extenders, but not at what length you can still use your GPU for gaming or GPGPU, where the connection between GPU and CPU is often used. So this test is quite useless for gaming purposes.