but ASUS specs claim only 6 PCIE on the AR version. Is it a typo on ASUS part?
Did you actually look at the specs? More like a typo on your part I'm thinking...
https://www.asus.com/Motherboards/PRIME-Z270-AR/specifications/Expansion Slots
2 x PCIe 3.0/2.0 x16 (x16 or dual x8)
1 x PCIe 3.0/2.0 x16 (max at x4 mode)
4 x PCIe 3.0/2.0 x1