It's important to note that, while it is true that you can't verify no changes have been made to the AOSP sources on the stock Pixel ROM, it's irrelevant because the Pixel ROM doesn't even claim to be AOSP -- it's a Google-flavored version of AOSP with the following "bonus features":
* Google cloud-synced replacements for the more barebones (but at least local) stock AOSP apps (which can be disabled, but not fully uninstalled): Google Calendar (instead of AOSP Calendar), Google Photos (AOSP Gallery), Gmail (AOSP Email), Google Keyboard (Android Keyboard), etc
* A few Pixel-exclusive Google apps not (yet) available on other Android phones: Pixel Launcher, Audio Recorder, etc
* The standard suite of Google system apps present even on most non-Pixel phones, like Play Store and Play Services, with special higher privileges that aren't available to other apps and can't be revoked
* The only option for system backup is to use Google Drive
It infuriates me that almost none of the stock apps (aside from Google Photos and maybe Gmail) allow you to use them without a Google account, by the way. On a Google Pixel, the stock note-taking app is Google Keep Notes, so you can't do something as simple as write a note locally, without sending it to Google servers, unless you install a third-party note-taking app. What the fuck.