Why is my security patch over a month old?
Since Android Oreo's official debut, Nougat is no longer the "current" Android version, therefore Google doesn't officially patch it. On the other hand, Oreo for N5X has already undergone 3 iterations corresponding to Aug., Sept. and Oct. patches.

Oreo is supposed to come to your device in one of the phased rollouts, but if you're eager to get it, consider backing up data, unlocking bootloader (if you haven't already), then either manually flashing a factory image or flashing a custom ROM (FYI, LineageOS has ported over the Sept. patch even though its official release is still on Nougat).