It appears after an extended wait, that Nexus 5X and Nexus 6P users enrolled in the Beta Program, are receiving an update which takes them from Android 7.1.1 Developer Preview 2 to the final build that started rolling out a while back.
The update, which users in the Android Beta Program Google+ community, as well as users over on reddit are reporting is 26MB in size for the Nexus 6P and 65MB in size for the Nexus 5X. The update to the final build (NMF26F) also includes the December security update. The update also removes the Beta popup that showed up after every reboot.
A quick check of devices here showed that opening Settings and then going to About phone > System update > Check for Update almost immediately got a prompt to download the update.
If you’ve been stuck on the Dev preview on your Nexus 5X or Nexus 6P check for an update.