It is not impossible that there is a backdoor in Pixel devices. None has been found. One difference between Google and Intel is that Google doesn't need a backdoor for the vast majority of people. They just use Google products like Google Andriod, Chrome and Gmail.

Researchers can monitor the network activity and radio activity of these phones and have yet to find a back door in Pixels. There are so many privacy and security issues that Graphene fixes but it is still a phone. You should research it for yourself. In the end you will have to choose who to trust. The most private option is no phone.