I think 170mm is just the length of the PCB itself.
I captured a high resolution pic of the back of the card and, with the known dimension of the length of the gold fingers insert, I estimate the length of the card (excluding the bracket) at 172.5mm (length of the backplate). Give or take a couple mm as margin of error, this does seem to indicate the PCB is 170mm long.
My card is 172mm from the face of the I/O shield/mounting bracket to the edge of the pcb. It matches up almost perfectly with the edge of the ITX motherboard. Measuring only the PCB/Backplate it is about 169mm.