the Z170 chipset provides 26 PCIe lanes in addition to the 16 PCIe lanes that the CPU provides.
So yes, the GPU slots use the 16 PCIe lanes that the CPU provides for that, and then the board designers get to allocate the other 26 to whatever else is on the motherboard.
This was presented by Intel as examples of how those chipset PCIe lanes could be allocated. These were suggestions, and most board designers followed this to some extent for USB and SATA ports.