The diesel emission test is an optical smoke test.The mot tester cannot override any rev limiter built into the cars fuel management system.

Yes,an optical probe is inserted into the exhaust tailpipe.It does not measure emissions similar to a petrol engine test.The only way an engine will rev to the red line ,when fitted with a limiter,is if the car is under load or in gear if an auto ,with the wheels rotating.This is not going to happen on an mot test.