The GrapheneOS developers like to talk as if relocking the bootloader is unique to Google Pixels and providing a custom key is unique to them.
But that's not true. Every OnePlus device in the last few years allows avb_custom_key as well, Xiaomi Mi A2 does too, probably many others. And ROMs like Pixel Experience publish their custom keys too.
Relocking the bootloader doesn't imply that verified boot is enabled or that that it's not broken for alternate OSes, as is the case for the devices you mentioned.
But that's not true. Every OnePlus device in the last few years allows avb_custom_key as well, Xiaomi Mi A2 does too, probably many others. And ROMs like Pixel Experience publish their custom keys too.