I know that the root user can access all files / commands on the system but can Can a root / su user assume any other uid?

The reason I ask is looking at http://nelenkov.blogspot.co.uk/2012/05/storing-application-secrets-in-androids.html it says a deamon commands "Allowed UIDs" are "anyone but root, vpn and wifi (Only keys created with the same UID are visible/accessible)" - which suggests in this case the creators uid has higher privilege than root - is this correct?