They can be machined to a certain minimum dimension but once they are too thin, no additional machining is allowed.
If you were told they cant be machined then ask why and if it is below the minimum spec then you'll have to buy new ones.

I saw the spec around the forum and the Bentley Manual, and I recall there was not much extra thickness there to allow a lot of machining.

Read your post about brake rotor dimensions. Was planning on having the rotors turned to clean them up for spring. Do you know why the Box S rotors cannot be machined?