You can tell it's truly secure and private because the Cellebrite leak says they can't break it (one of very few!) and some governments assume you're a drug dealer if you use it. My next phone will run GrapheneOS.
Marketing is known to lede a lot of bullshit. Celebrite saying "We tried our stuff and it didn't work" is very likely more about obscurity than focused effort. Unless you know that celebrite has been specifically hired to break into a GrapheneOS device and put as much effort into it as they do into standard Android and iOS and failed, it means nothing. It's like the old thing about MacOS 9 being more secure than Windows.