treequell That is incorrect. Please look at: https://attestation.app/about#device-support
If a device launched with Android 8, it can be verified by Auditor if it's running the Stock OS, provided that Auditor adds explicit support for said model.
As far a device doing the auditing, any device running Android 8 or later will do just fine, not just the ones listed there.
It can also of course verify GrapheneOS (in addition to Stock OS) for the devices that GrapheneOS supports.
With all of that said, @de0u - this issue may provide some clarity and insight into why more devices are not being added at the moment:
TL;DR is that to my understanding it's too complex and time consuming to do on a large scale as things currently stand. A new approach is needed and elaborated on a little bit in the issue above.