There is a nice little post here from one of the GrapheneOS project members/moderators.

https://discuss.grapheneos.org/d/10150-not-your-average-why-...

the tl;dr:

- There has never been any evidence found of any hardware backdoors in Pixel devices. - Those would be quickly noticed. - Those are poor and inefficient methods for any sort of data collection.

To answer your question they are not "just hoping for the best".