That is correct. The Thunderbolt controller has two DisplayPort sink ports which are wired to the GPU, in addition to the PCIe 3.0 4x interface which goes to a root port in the CPU. So to actually max out the 40 Gbit/s, you need to drive external displays and also max out the PCIe 3.0 4x interface. Which is another way to say that 8 Gbit/s are reserved for DisplayPort.