> so you can attest

This isn't about you attesting anything though. It's about corporations attesting that your device is 100% corporate owned. Can't have you running software that impacts their bottom line after all.

GrapheneOS could be the most secure operating system to ever exist, it doesn't matter to the corporation because it's still under your control. When they say "security", they mean "the corporation's security against the user", not "the user's security against the hostile world out there".

They have to come and outright say "we block this because we don't control it". They can't say "we block this because it isn't secure" because it's obviously very secure.