Historically, some very large Android OEMs based in China and selling in China also provided bootloader unlocking support in their devices including Xiaomi https://github.com/zenfyrdev/bootloader-unlock-wall-of-shame... and Oppo https://github.com/zenfyrdev/bootloader-unlock-wall-of-shame...

It doesn't have anything directly to do with GrapheneOS, and would have enabled custom OS support of any Android distribution.

Xiaomi claims to provide unlocking, but doesn't actually. I don't know about Oppo.