Their marketing slides include GOS, and they have since 2021 I think was just said "can't access that".

I would imagine it's a very high priority if you have powerful targets. They're not gonna be running an off-the-shelf android phone.

Considering they mention it on their marketing materials, they definitely did bother

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.