No, the kernel knows what it's "real" UID and GID are. It even knows what unmapped UIDs and GIDs are. I haven't tried to access the root keychain inside a rootl

No, the kernel knows what it's "real" UID and GID are. It even knows what unmapped UIDs and GIDs are. I haven't tried to access the root keychain inside a rootless container, but if it does work I would consider that to be a vulnerability in the kernel.