When companies like Google, are working on their next big version of Android (or any other platform), they go through numerous builds before the final "stable" build is rolled out to everyone. Typically, these builds are sent out to a group of testers - typically employees - to check out the update and make sure everything is working fine. It appears that a Nexus 6P user mistakenly got a newer build of Android 7.0 Nougat for his Nexus 6P. This new build was version NRD90M, which is different and newer than the Developer Preview 5 which is build NPD90G.
According to the user, who posted about it over on Reddit, he was getting out of the Android Beta Program, due to his Nexus 6P having battery issues. What is supposed to happen when you un-enroll from the Android Beta Program is, you are supposed to get a new OTA which will take you back to Android 6.0.1 Marshmallow. Instead, he was greeted with this "CONFIDENTIAL INTERNAL ONLY" build on his Nexus 6P. The OTA also noted that this is a "confidential Googlers-only OTA". The OTA weighs in at about 49.5MB, so it's not a large one, and probably doesn't change a whole lot on the Nexus 6P - likely bug fixes and such.
A few changes that he noticed once he installed the OTA was the fact that the "Android Beta Program" tile in Quick Settings has disappeared. Which shows further proof that we are nearing the release of the final version of Android 7.0 Nougat. Unfortunately, the user was not able to grab the OTA link, so others won't be able to download and install it onto their own Nexus 6P. The update also brings the latest security patch up to August 5th, 2016. Which means we should see this security update next week (as August 5th is next Friday). Not a big surprise there though, seeing as Google is rolling out security patches each month now.
Google is expected to release the final build of Android 7.0 Nougat fairly soon, since they have already pushed out the final developer preview. Google initially said that they would be pushing out Android N in the third quarter, which means anytime now. Of course, with that final version of Nougat, we'll also be getting new Nexus hardware. And with all of the recent leaks, it looks like that is right around the corner.