I can't figure out the "de-Google by using their hardware" logic either. With SOC tech being cheap, it would be trivial for them to still have ways of snooping. Not to mention the financial benefit. 

 Because they don't think much.

I have an article about how fucked up phones are these days -

nostr:naddr1qvzqqqr4gupzqamkcvk5k8g730e2j6atadp6mxk7z4aaxc7cnwrlkclx79z4tzygqqx8q6tcv4krsun9we5k2acm2j2ve 

 The tools cops use to crack phones don't work on GrapheneOS, that's the main thing I think.

The devs of GrapheneOS have great documentation, a lot of questions could be answered reading that 
https://grapheneos.org/features
They have forums too, if you have more questions. 
https://discuss.grapheneos.org/